Display apparatus with a variable aperture stop means on each side of the modulator

ABSTRACT

A display apparatus includes a light source, a modulating device for modulating light emitted from the light source, a modulating device driving unit for driving the modulating device, a first aperture stop unit arranged between the light source and the modulating device to limit a bundle of rays to be incident on the modulating device from the light source, the first aperture stop unit having an aperture with a variable size, a display screen onto which exit light from the modulating device is projected, a projection optical system for projecting the exit light from the modulating device onto the display screen, a second aperture stop unit arranged between the modulating device and the projection optical system to limit a bundle of rays to be incident on the projection optical system from the modulating device, the second aperture stop unit having an aperture with a variable size, a photosensor for detecting the display luminance on the display screen, and an aperture control unit for controlling the size of the aperture of at least one of the first and second aperture stop units on the basis of a display luminance signal from the photosensor.

This is a division of application Ser. No. 08/363,776, filed Dec. 27,1994 pending.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a display apparatus for displayingimages on the basis of picture signals and, more particularly, to adisplay apparatus for projecting light from a light source onto a screenvia a modulating device.

2. Description of the Related Art

A plasma display panel and a modulating device are presently attractingattention as compact, light-weight flat display apparatuses which canreplace CRT display apparatuses. These flat display apparatuses areroughly classified into two categories; i.e., a self-luminescing typedisplay apparatus which emits light by itself in a display operation anda transmittance control type display apparatus which controls thetransmittance of incident light from a separate light source in adisplay operation. For example, a plasma display panel belongs to theself-luminescing type, and a modulating device belongs to thetransmittance control type. In particular, the modulating device isconsidered as the most promising display apparatus of the nextgeneration, so the research and development of the modulating device arebeing done in various application fields.

As represented by a twisted nematic type modulating device introduced inLiquid Crystal Device Handbook, a general modulating device ischaracterized in that light which is linearly polarized by a polarizingplate is input to a liquid crystal layer having birefringence or opticalrotatory power. Unfortunately, modulating devices of this sort have thedrawback that the quantity of light obtained from the light source isreduced to about one half when the light passes through the polarizingplate.

Recently, a modulating device requiring no such polarizing plate asdiscussed above has been developed. This modulating device has either apolymer dispersion type liquid crystal layer in which a liquid crystalmaterial is contained in a polymer resin, or a fine-particle dispersiontype liquid crystal layer in which fine particles are contained in aliquid crystal material, between a pair of transparent electrodesubstrates, or between a transparent substrate and a reflectiveelectrode substrate. This modulating device functions as a scatteringtype modulating device which modulates the spatial propagation directionof incident rays on the liquid crystal layer. In this device the useefficiency of the light from the light source is improved in comparisonwith that of an apparatus using a polarizing plate.

A modulating device having the polymer dispersion type liquid crystallayer is set in a milky white, light-scattering state, in which incidentlight rays are scattered, in a pixel region between electrodes appliedwith no voltage, and in a transparent, light-transmitting state, inwhich incident light rays are not easily scattered, in a pixel regionbetween electrodes applied with a voltage. Therefore, thelight-scattering property of each pixel region is so controlled that theintensity of the transmitted light or the reflected light changes inaccordance with a picture signal. Consequently, one of the transmittedlight and the reflected light is guided to the screen by a projectionoptical system.

The function of a modulating device having the fine-particle dispersiontype liquid crystal layer is basically identical with that of themodulating device having the polymer dispersion type liquid crystallayer.

As a display apparatus of another type, a micro mirror device (DMD) isintroduced in SID93 Digest, from page 1,012. In the micro mirror device,the angles of micro mirrors arranged in a two-dimensional matrix mannerare independently changed to control the direction of reflected rays,and the light reflected in a desired direction is guided to the screenby a projection optical system. The micro mirror device also functionsin the same fashion as the modulating device having the fine-particleliquid crystal layer or the polymer dispersion type liquid crystal layerin that the spatial propagation direction of the incident light rays ismodulated.

FIG. 1 schematically illustrates the arrangement of a conventionalprojection display apparatus. In FIG. 1, a light source unit 11 isconstituted by a lamp 12 as a light source and a collimator opticalsystem 13 for collimating light from the lamp 12 into parallel rays. Ascattering type modulating device 14 has a function of two-dimensionallymodulating the spatial propagation direction of the incident parallelrays from the collimator optical system 13. The scattering typemodulating device 14 is, e.g., a polymer dispersion type modulatingdevice. A driver 20 drives the modulating device 14 in accordance with apicture signal. A projection optical system 16 includes an aperture stopunit 15 for extracting light rays within a certain fixed angle rangefrom the light transmitted through the modulating device 14. Theprojection optical system 16 projects the extracted transmitted lightonto a screen 17. In this manner, an image is displayed on the screen 17with a light intensity distribution corresponding to the picture signal.

The contrast and the luminance of the displayed image depend upon theangle distribution of light rays constituting the exit light from themodulating device 14 and used in the display. The contrast improves asthe angle distribution decreases, and the luminance improves as theangle distribution increases. That is, the contrast and the luminance ofthe displayed image are contrary to each other.

Jpn. Pat. Appln. KOKAI Publication No. 5-216004 or 5-188345, forexample, has disclosed a technique by which the relationship between thecontrast and the luminance of a displayed image is optimized inaccordance with the luminance of the use environment by using anarrangement in which the aperture size of the aperture stop unit 15 forstopping down the exit light rays from the modulating device 14 is madevariable.

Unfortunately, it is difficult to improve both the contrast and theluminance to respective satisfactory degrees simply by making theaperture size variable in accordance with the luminance of the useenvironment.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a display apparatuscapable of displaying images easier to see in accordance with theenvironment in which a screen is placed.

It is another object of the present invention to provide a displayapparatus capable of accurately reproducing the gradation of an image.

It is still another object of the present invention to provide a displayapparatus capable of displaying high-quality images regardless of theenvironmental temperature condition.

According to one aspect of the present invention, there is provided adisplay apparatus comprising a light source, a modulating device formodulating light emitted from the light source, liquid crystal drivingmeans for driving the modulating device, first aperture stop means,arranged between the light source and the modulating device, forlimiting a bundle of rays to be incident on the modulating device fromthe light source, the first aperture stop means having an aperture witha variable size, a display screen onto which exit light from themodulating device is projected, a projection optical system forprojecting the exit light from the modulating device onto the displayscreen, second aperture stop means, arranged between the modulatingdevice and the projection optical system, for limiting a bundle of raysto be incident on the projection optical system from the modulatingdevice, the second aperture stop means having an aperture with avariable size, a photosensor for detecting a display luminance on thedisplay screen, and aperture control means for controlling the size ofthe aperture of at least one of the first and second aperture stop meanson the basis of a display luminance signal from the photosensor.

According to another aspect of the present invention, there is provideda display apparatus comprising a light source, a modulating device formodulating light emitted from the light source, liquid crystal drivingmeans for driving the modulating device, first aperture stop means,arranged between the light source and the modulating device, forlimiting a bundle of rays to be incident on the modulating device fromthe light source, the first aperture stop means having an aperture witha variable size, a display screen onto which exit light from themodulating device is projected, a projection optical system forprojecting the exit light from the modulating device onto the displayscreen, second aperture stop means, arranged between the modulatingdevice and the projection optical system, for limiting a bundle of raysto be incident on the projection optical system from the modulatingdevice, the second aperture stop means having an aperture with avariable size, a photosensor for detecting a display luminance on thedisplay screen, and aperture control means for controlling the size ofthe aperture of at least one of the first and second aperture stop meanson the basis of a picture luminance signal supplied from the liquidcrystal driving means to the modulating device and a display luminancesignal from the photosensor.

According to still another aspect of the present invention, there isprovided a display apparatus comprising a light source, a modulatingdevice for modulating light emitted from the light source, liquidcrystal driving means for driving the modulating device, first aperturestop means, arranged between the light source and the modulating device,for limiting a bundle of rays to be incident on the modulating devicefrom the light source, the first aperture stop means having an aperturewith a variable size, a display screen onto which exit light from themodulating device is projected, a projection optical system forprojecting the exit light from the modulating device onto the displayscreen, second aperture stop means, arranged between the modulatingdevice and the projection optical system, for limiting a bundle of raysto be incident on the projection optical system from the modulatingdevice, the second aperture stop means having an aperture with avariable size, aperture control means for controlling the size of theaperture of at least one of the first and second aperture stop means,and liquid crystal driving means for supplying to the modulating devicea picture signal controlled on the basis of the size of the aperture,which is controlled by the aperture control means, of at least one ofthe first and second aperture stop means.

According to still another aspect of the present invention, there isprovided a display apparatus comprising a light source, a modulatingdevice for modulating light emitted from the light source, liquidcrystal driving means for driving the modulating device, first aperturestop means, arranged between the light source and the modulating device,for limiting a bundle of rays to be incident on the modulating devicefrom the light source, the first aperture stop means having an aperturewith a variable size, a display screen onto which exit light from themodulating device is projected, a projection optical system forprojecting the exit light from the modulating device onto the displayscreen, second aperture stop means, arranged between the modulatingdevice and the projection optical system, for limiting a bundle of raysto be incident on the projection optical system from the modulatingdevice, the second aperture stop means having an aperture with avariable size, a temperature sensor arranged near the modulating device,and picture signal control means for controlling a picture signalsupplied from the driving means to the modulating device on the basis ofa temperature signal from the temperature sensor.

According to still another aspect of the present invention, there isprovided a display apparatus comprising a light source, a modulatingdevice for modulating light emitted from the light source, liquidcrystal driving means for driving the modulating device, first aperturestop means, arranged between the light source and the modulating device,for limiting a bundle of rays to be incident on the modulating devicefrom the light source, the first aperture stop means having an aperturewith a variable size, a display screen onto which exit light from themodulating device is projected, a projection optical system forprojecting the exit light from the modulating device onto the displayscreen, second aperture stop means, arranged between the modulatingdevice and the projection optical system, for limiting a bundle of raysto be incident on the projection optical system from the modulatingdevice, the second aperture stop means having an aperture with avariable size, aperture control means for controlling the size of theaperture of at least one of the first and second aperture stop means,and compensating means for compensating for a change in a drivingvoltage-modulated light intensity characteristic of the modulatingdevice caused in correspondence with the size of the aperture, which iscontrolled by the aperture control means, of at least one of the firstand second aperture stop means.

According to still another aspect of the present invention, there isprovided a display apparatus comprising a light source, a modulatingdevice for modulating light emitted from the light source, liquidcrystal driving means for driving the modulating device, first aperturestop means, arranged between the light source and the modulating device,for limiting a bundle of rays to be incident on the modulating devicefrom the light source, the first aperture stop means having an aperturewith a variable size, a display screen onto which exit light from themodulating device is projected, a projection optical system forprojecting the exit light from the modulating device onto the displayscreen, second aperture stop means, arranged between the modulatingdevice and the projection optical system, for limiting a bundle of raysto be incident on the projection optical system from the modulatingdevice, the second aperture stop means having an aperture with avariable size, aperture control means for controlling the size of theaperture of at least one of the first and second aperture stop means,light intensity setting means for setting at least two light intensitiesI, a photosensor for detecting display luminances L on the screen whichcorrespond to the light intensities I, environment analyzing means forcalculating a projection coefficient q and an environmental luminance L₀(L₀ is a luminance generated on the display screen resulting from lightfrom an environment in which the display apparatus is placed, i.e.,environmental luminance) by substituting the light intensities I and thedetected display luminances L into an equation L=qI+L₀, data storagemeans for calculating a contrast on the display screen from the equationsolved for the projection coefficient q and the environmental luminanceL₀ and storing data indicating the size of the aperture, by which thecontrast is maximized, of at least one of the first and second aperturestop means, and processing means for specifying the size of the apertureof at least one of the first and second aperture stop means, the data ofwhich is stored in the data storage means and by which the contrast ismaximized, and determining the size as an optimum value.

Additional objects and advantages of the invention will be set forth inthe description which follows, and in part will be obvious from thedescription, or may be learned by practice of the invention. The objectsand advantages of the invention may be realized and obtained by means ofthe instrumentalities and combinations particularly pointed out in theappended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of the specification, illustrate presently preferred embodiments ofthe invention and, together with the general description given above andthe detailed description of the preferred embodiments given below, serveto explain the principles of the invention.

FIG. 1 is a schematic view showing the arrangement of a conventionalprojection display apparatus;

FIG. 2 is a view showing an example of a collimator light source forobtaining parallel light rays from a lamp;

FIG. 3 is a view showing incident light rays to a condenser lens shownin FIG. 2;

FIG. 4 is a view showing the relationship between the angle distributionof an incident light beam to a modulating device and the angledistribution of an exit light beam from the modulating device;

FIG. 5 is a graph showing the relationship between the intensity of anexit light beam from the modulating device and the contrast of an imagemeasured in correspondence with the converging angle when the image isdisplayed on a screen placed in an illuminated environment;

FIG. 6A is a block diagram showing the arrangement of a projectiondisplay apparatus for explaining a fundamental concept of the presentinvention;

FIG. 6B is a block diagram showing the arrangement of a projectiondisplay apparatus according to the first aspect of the presentinvention;

FIGS. 7A and 7B are views each showing the shape of the aperture of anaperture stop;

FIGS. 8A to 8C are views showing various aperture diameters of theaperture stop;

FIG. 9 is a flow chart for explaining the operation of the displayapparatus shown in FIG. 6;

FIG. 10 is a flow chart showing the details of the processing operationin FIG. 9;

FIG. 11 is a flow chart for explaining the operation of anotherembodiment of the projection display apparatus according to the firstaspect of the present invention;

FIG. 12 is a block diagram showing the arrangement of a projectiondisplay apparatus according to the second aspect of the presentinvention;

FIG. 13 is a graph showing the relationship between the two inputsignals to a controller in FIG. 12 and the angle of collection ofaperture stops;

FIG. 14 is a circuit diagram showing the details of an average picturelevel (APL) detector in FIG. 12;

FIG. 15 is a view showing the collector input to a transistor, theemitter output from a buffer transistor, and the emitter output fromanother buffer transistor of the circuit illustrated in FIG. 14;

FIG. 16 is a circuit diagram showing another example of the APLdetector;

FIG. 17 is a view showing the collector output from a transistor and theemitter output from another transistor of the circuit in FIG. 16;

FIG. 18 is a graph showing the relationship between the angle ofcollection and the display characteristic;

FIG. 19 is a graph showing the effective display characteristics in abright environment;

FIG. 20 is a graph showing the driving voltage-transmitted lightintensity characteristic of a modulating device in a display apparatusaccording to the third aspect of the present invention;

FIG. 21 is a graph for explaining that the transmittance of themodulating device in the display apparatus according to the third aspectof the present invention shifts from a desired value due to a change inthe angle of collection of an aperture stop;

FIG. 22 is a view showing the distribution of incident light beams tothe modulating device and the distribution of exit light beams from themodulating device;

FIG. 23 is a view showing the light scattering property of themodulating device;

FIGS. 24A to 24C are views showing the state in which an effectiveportion of the exit light beam, FIG. 23, which contributes to a display,changes in dependence on the angle of collection;

FIG. 25 is a view showing the arrangement of the projection displayapparatus according to the third aspect of the present invention;

FIG. 26 is a block diagram showing the details of the arrangement of agamma-correcting circuit in FIG. 25;

FIG. 27 is a graph showing the temperature dependence of alight-modulating device;

FIG. 28 is a view showing the arrangement of a projection liquid crystaldisplay apparatus according to the fourth aspect of the presentinvention;

FIG. 29 is a plan view showing the arrangement of a modulating deviceused in the display apparatus shown in FIG. 28;

FIG. 30 is a sectional view showing the arrangement of the modulatingdevice used in the display apparatus in FIG. 28;

FIG. 31 is a perspective view showing the position of a temperaturesensor in the display apparatus in FIG. 28;

FIG. 32 is a circuit diagram showing a practical circuit of thetemperature sensor;

FIG. 33 is a view showing a driving voltage supply circuit in thedisplay apparatus in FIG. 28;

FIG. 34 is a graph showing the relationship between the change rate of acontrast and the recognition probability;

FIG. 35 is a block diagram showing the arrangement of a simplifiedprojection display apparatus according to the sixth aspect of thepresent invention;

FIG. 36 is a flow chart showing the processing of automaticallydetermining the angle of collection after a power supply is turned on inthe display apparatus shown in FIG. 35;

FIG. 37 is a block diagram showing the arrangement of another embodimentof the simplified projection display apparatus;

FIG. 38 is a flow chart showing the processing of automaticallydetermining the angle of collection in the display apparatus shown inFIG. 37;

FIG. 39 is a perspective view showing a rear projection type displayapparatus;

FIG. 40 is a sectional view showing the rear projection type displayapparatus in FIG. 39;

FIG. 41 is a view showing the arrangement of a projection liquid crystaldisplay apparatus according to the fifth aspect of the presentinvention;

FIG. 42 is a view showing a driving voltage supply circuit in thedisplay apparatus in FIG. 41; and

FIG. 43 is a sectional view showing the arrangement of the modulatingdevice used in the display apparatus in FIG. 41.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In a display apparatus according to the first aspect of the presentinvention, in an aperture adjustment mode a control means causes amodulating device to produce modulated light under predeterminedconditions. With this modulated light projected onto a screen, thecontrol means detects the luminance on the screen. In accordance withthis detected luminance, the control means adjusts the angle ofcollection, i.e., the aperture size of at least one of first and secondaperture stop means such that the contrast and the luminance of thedisplayed image are set to their respective optimum values. This controlof the aperture size is based on the finding obtained by the presentinventors that the luminance and the contrast of a displayed imagelargely depend upon the angle distribution of a light beam incident onthe modulating device.

By using the luminance on the screen, which reflects the angledistribution of a light beam incident on the modulating device, as afactor for determining an optimum value of the angle of collection, itis possible to obtain displayed images easier to see under theenvironment in which the screen is placed. It is also possible toautomatically correct the angle of collection to an optimum value in theaperture adjustment mode even if the luminance of the room has changed.

To allow a better understanding of the present invention, therelationship between the luminance and the contrast of an imagedisplayed on a screen will be explained below.

FIG. 2 shows an example of a collimator light source for obtainingparallel light rays from a lamp. This collimator light source isconstituted by a lamp 50, a focusing lens system 51, an aperture stop52, and a collimator lens 53, all of which are arranged along theoptical axis.

One characteristic feature of the existing lamp 50 is that the lamp isnot a point illuminant which emits light from a single point. Thefocusing lens system 51 focuses light, which is emitted from the surfaceof the lamp 50 with a certain area, at the aperture stop 52 into acircular lamp image 60 with a radius R. The aperture stop 52 limits thebeam area of the lamp image 60 in a plane perpendicular to the opticalaxis. Therefore, the lamp image 60 can be regarded as a surfaceilluminant 62 with an area which is determined by a radius r of theaperture stop 52. The light beam from this surface illuminant 62 entersa scattering type modulating device 54 through the collimator lens 53.However, the two-dimensional spread of the illuminant 62 gives an angledistribution to the incident rays to this modulating device 54.

FIG. 3 shows the incident light rays to the collimator lens 53. Thiscollimator lens 53 is a convex lens having a focal point in the centerof the surface illuminant 62 and having a sufficiently small aberration.The collimator lens 53 is used to form parallel rays from the light raysemitted from the surface illuminant 62. Assuming that the center and theend portion of the surface illuminant 62 are a point a and a point b,respectively, all homocentric light rays propagating from the point a tothe collimator lens 53 become parallel to the optical axis after beingtransmitted through the collimator lens 53. In contrast, all homocentriclight rays traveling from the point b to the collimator lens 53 form aconstant angle θ with the optical axis after being transmitted throughthe collimator lens 53. This angle θ is in proportion to the distancebetween the points a and b, i.e., the radius r of the lamp image 62. Thecollimator lens 53 outputs, toward the modulating device, a light beamwhose angle distribution is θ in correspondence with the entire surfaceilluminant 62. If the range of θ is not so wide, an angle distributionΩ_(i) of the incident light beam to the modulating device can berepresented as a solid angle by Equation (1) below: ##EQU1##

Since the angle θ and the radius r of the lamp image are in proportionas discussed above, the angle distribution Ω_(i) of the incident lightbeam and the area of the lamp image 62 are proportional. Thisdemonstrates that, assuming the intensity of the light beam transmittedthrough the aperture stop 52 and incident on the modulating device isI_(i), within the range of r<R the angle distribution Ω_(i) and the beamintensity I_(i) tend to monotonically increase as the radius r of theaperture stop 52 increases.

The display characteristics of the scattering type modulating device onwhich the light beam with the intensity I_(i) is incident will bedescribed below. FIG. 4 shows the relationship between the angledistribution Ω_(i) of the incident light beam to the modulating deviceand the angle distribution Ω₀ of the exit light beam from the modulatingdevice. For the sake of simplicity, assume that the incident light beamis uniform over the range of the angle distribution Ω_(i) and does notexist outside this range. Assume also that the modulating deviceuniformly scatters the incident light beam in the scattering state. Thisscattering power is represented by Ω_(p) when the angle distribution ofan exit light beam obtained in correspondence with an incident lightbeam having no angle distribution is Ω₀.

If this angle distribution Ω_(p) is sufficiently large with respect tothe angle distribution Ω_(i) of the incident light beam, the displaycharacteristic of the modulating device holds the followingrelationships with the aperture stop on the light source side and theaperture stop on the projection side.

The angle distribution Ω_(i) of the incident light beam represents thestate of the aperture stop on the light source side and satisfiesEquation (2) below with respect to a angle of collection Ω_(A1) of theaperture stop:

    Ω.sub.i ≃Ω.sub.A1                (2)

Assuming the angle distribution Ω₀ of the exit light beam from themodulating device in the non-scattering state is Ω_(ON), this angledistribution Ω_(0ON) directly reflects the angle distribution Ω_(i) ofthe incident light beam as indicated by Equation (3) below:

    Ω.sub.0 =Ω.sub.0ON =Ω.sub.i              (3)

Assuming the angle distribution Ω₀ of the exit light beam from themodulating device in the scattering state is Ω_(0OFF), this angledistribution Ω_(0OFF) takes a superposed form of the angle distributionΩ_(p) indicating the scattering power of the modulating device and theangle distribution Ω_(i) of the incident light beam, as illustrated inFIG. 4. From the relation Ω_(i) <Ω_(p), the angle distribution Ω_(0OFF)can be approximated to Ω_(p) as represented by Equation (4) below:

    Ω.sub.0 =Ω.sub.0OFF ≈Ω.sub.i     (4)

The exit light beam discussed above is uniform over the range of theangle distribution Ω₀, and only rays within the range of a fixed angleof collection Ω_(A2) which is determined by the aperture stop on theprojection side are extracted and displayed. Equations (5) and (6) shownbelow indicate the relation of the intensity I₀ of the exit light beamto the intensity I_(i) of the incident light beam.

    I.sub.0 =(Ω.sub.A2 /Ω.sub.0)·I.sub.i (Ω.sub.A2 <Ω.sub.0)                                           (5)

In this case the intensity I₀ of the exit light beam is determined bythe ratio of Ω₀ to Ω_(A2).

    I.sub.0 =I.sub.i (Ω.sub.A2 ≧Ω.sub.0)    (6)

Contrast CR is obtained as the ratio of an exit beam intensity I_(ON)when the modulating device is in the scattering state to an exit beamintensity I_(OFF) when the device is in the non-scattering state.Generally, Ω_(A2) <Ω_(p). Therefore, the contrast CR is given byEquation (7) or (8) depending on the magnitude relationship betweenΩ_(A2) and Ω_(A1). In the case of ##EQU2## In the case of ##EQU3##

That is, assuming that a larger one of the angle of collection Ω_(A1) ofthe aperture stop on the light source side and the angle of collectionΩ_(A2) of the aperture stop on the projection side is represented byΩ_(A), the contrast CR is given by the ratio of the angle distributionΩ_(p) indicative of the scattering power to the angle of collectionΩ_(A), as shown by Equation (9) below:

    CR=Ω.sub.p /Ω.sub.A                            (9)

Consider the relationship between the magnitudes of the angle ofcollections Ω_(A1) and Ω_(A2) which determine the contrast and theluminance of a displayed image, i.e., the exit beam intensity I_(ON). IfΩ_(A1) ≦Ω_(A2), the exit beam intensity I_(ON) of a white display isconstant independently of Ω_(A2) from Equation (6). In this case thecondition under which the contrast is a maximum is Ω_(A1) =Ω_(A2) fromEquation (9). If Ω_(A1) ≧rΩ_(A2), the contrast CR=Ω_(p) /Ω_(A1) fromEquation (7), and I_(ON) =I_(i) Ω_(A2) /Ω_(A1) from Equation (5).Therefore, the exit beam intensity I_(ON) is a maximum when Ω_(A2)=Ω_(A1) (=Ω_(i)). From these two relationships, the condition underwhich the highest exit beam intensity is obtained at an arbitrarycontrast and the best contrast is obtained at an arbitrary luminance isΩ_(A1) =Ω_(A2). As discussed above, the condition by which the bestdisplay characteristics are obtained is that the angle of collectionΩ_(A1) of the aperture stop on the light source side is in agreementwith the angle of collection Ω_(A2) of the aperture stop on theprojection side.

Even under the optimum condition discussed above, although the luminanceof a displayed image, i.e., the exit beam intensity I_(ON) (=I_(i))increases as the radius r of the aperture stop increases, the contrastCR of the displayed image decreases with increasing radius. Thisindicates that it is not possible to improve both I_(ON) and CR bychanging these factors by adjusting the size of the aperture stop.

The foregoing is a qualitative examination, so a detailed examinationmust be performed for individual optical systems in accordance with therespective schemes of the systems, lamp property, or the like. However,the above basic behavior that the luminance and the contrast of adisplayed image trade off each other is a common characteristic ofoptical systems using a modulating device for modulating the spatialpropagation direction of light, such as a polymer dispersion typemodulating device, a fine-particle dispersion type modulating device,slant field effect liquid crystal diffraction grating, (Japanese PatentApplication Nos. 6-298496, 6-172935) or a DMD.

Consider the contrast of an image displayed on a screen by the operationof a projection display apparatus using a polymer dispersion type liquidcrystal as a scattering type modulating device. The contrast of thisdisplayed image is influenced by the surrounding environment of thescreen. Therefore, while an image was displayed on the screen placed inan illuminated room, the intensity of an exit light beam from themodulating device and the contrast of the image were measured incorrespondence with the angle of collection of an aperture stop. FIG. 5shows the intensity of the exit light beam from the modulating deviceand the contrast of the image measured in correspondence with the angleof collection. The relationship in FIG. 5 shows that an optimum value bywhich the maximum contrast is given exists within the adjustable rangeof the angle of collection.

When an image is displayed on the screen by the operation of the displayapparatus, luminance L of the screen is the sum of luminances L_(ON) orL_(OFF) of the displayed images and luminance L₀ of the background whichdepends upon the environment in which the screen is placed. Therefore,an actual contrast CR_(room) is represented using the luminances L_(ON)and L_(OFF) by Equation (10) below:

    CR.sub.room =(L.sub.ON +L.sub.0)/(L.sub.OFF +L.sub.0)      (10)

The luminance L₀ of the background is a value obtained by multiplyingthe intensities of, e.g., the illuminating light in the room in whichthe screen is set and external light entering through windows, by one ofthe light transmittance and the light reflectance that are determined bythe display form (a light transmitting type or a light reflecting type)of the screen. This L₀ is not negligible since it has an effect ofsignificantly decreasing the original contrast CR obtained by Equation(9).

If the value of L₀ is sufficiently small, the contrast exhibits adependence of 1/Ω_(A) close to the original slope and improves as theangle of collection Ω_(A) decreases. If the value of L₀ is sufficientlylarge, the maximum luminance becomes dominant over the contrast; thatis, the contrast tends to increase as the angle of collection Ω_(A)increases.

Consider, therefore, a contrast CR_(room) obtained in correspondencewith the aperture stop angle of collection Ω_(A) when L₀ takesintermediate values. The relationship between the aperture stop angle ofcollection Ω_(A), the intensity I of projected light from the displayapparatus, and the luminance L of the screen is given by using positiveproportionality factors k and q by Equation (11):

    L=KΩ.sub.A =qI                                       (11)

Equation (12) below indicates the actual contrast CR_(room) when theluminance L₀ of the background is taken into account: ##EQU4## When L₀=0, the contrast improves as the angle of collection Ω_(A) decreases, asdiscussed above. Equation (13) below represents the condition requiredto obtain the maximum contrast when L₀ takes a finite value larger than0: ##EQU5##

If there is a point at which the contrast reaches a maximum, the pointis dependent upon the luminance L₀ of the background and the angle ofcollection Ω_(A) of the aperture stop. If the angle of collection Ω_(A)has a realizable solution in Equation (13), this solution is an optimumvalue which allows the contrast CR_(room) to be a maximum.Unfortunately, the upper limit of the angle of collection Ω_(A) of theaperture stop is essentially determined by the size of an actual lightsource. The upper-limit value of this adjustable range is present. WhenL₀ has exceeded a certain fixed value, the maximum value of the angle ofcollection within the adjustable range becomes an optimum value in orderthat the contrast becomes maximum within the adjustable range. If thevalue of L₀ is almost 0, the contrast improves as the angle ofcollection Ω_(A) decreases. If the angle of collection Ω_(A) is toosmall, however, displayed images become dark and difficult to see. Inthese instances the angle of collection Ω_(A) is set at a minimum valuewithin the range over which displayed images can be readily seen.

There will be described a fundamental concept of the present inventionprior to describing a projection display apparatus according to thepresent invention.

FIG. 6A shows the arrangement of this projection display apparatus. Theprojection display apparatus comprises an optical system having aspheroidal mirror 101, a light source lamp 102, a collimator lens 103,an electric aperture stop 104, a scattering type liquid crystal panel108, a field lens 109, projection lenses 110, and another electricaperture stop 111. The light from the lamp 102 enters the collimatorlens 103 directly and after being reflected by the mirror 101. Thecollimator lens 103 outputs this incident light as parallel rays to thescattering type liquid crystal panel 108. The scattering type liquidcrystal panel 108 includes a liquid crystal layer, in which a liquidcrystal material is dispersed in a polymer resin, as a light-modulatinglayer between a pair of transparent electrode substrates. The scatteringtype liquid crystal panel 108 is driven by a modulating device driver107 as a light-modulating device which modulates the spatial propagationdirection of light by using this light-modulating layer. The modulatedlight from the scattering type liquid crystal panel 108 is incident onthe projection lenses 110 through the field lens 109. The projectionlenses 110 project the modulated light onto a reflection screen SC. Thatis, the basic display concept of this projection display apparatus isidentical with that of the projection display apparatus illustrated inFIG. 1.

As shown in FIG. 6A, this display apparatus includes the two electricaperture stops 104 and 111. The electric aperture stop 104 narrows thebundle of rays entering the collimator lens 103. The electric aperturestop 111 narrows the bundle of rays projected from the projection lenses110. Each of the electric aperture stops 104 and 111 has an internalservo motor M which is controlled by a controller 120. The aperturesize, i.e., the shape of the aperture of each aperture stop is adjustedby the action of the servo motor. The controller 120 operates theaperture stops 104 and 111 on the basis of an input signal A from aluminance signal smoothing circuit 140, and input signal B from adecoder 121, thereby controlling the distribution of the incident lightbeam to the scattering type liquid crystal panel 108 and the exit beamangle range which contributes to a display. The decoder 121 receives acontrol signal transmitted from an external infrared remote controllerand decodes the received signal to obtain the signal B.

FIG. 13 shows the relationship between the input signals A and B to thecontroller 120 and the angle of collection of the aperture stops. Thestate of each aperture stop is represented by the angle at which theexit light beam from the scattering type liquid crystal panel 108 passesthrough the aperture stop, i.e., the angle of collection. The angle ofcollection of the aperture stop 111 is so set as to be variable withinthe range from 8.6×10⁻³ to 1.1×10⁻³ sr. The aperture stop 104 is socontrolled that a light beam within the same angle range is incident onthe scattering type liquid crystal panel 108.

The input signal A to the controller 120 indicates the time averageintensity of a luminance signal contained in a picture signal. The inputsignal A is generated by the luminance signal smoothing circuit 140. Asin FIG. 12, this luminance signal smoothing circuit 140 is composed of aluminance signal blanking level (black level) detector 140A and an RCintegrator 140B. A time constant RC of the integrator 140B can bealtered by adjusting a resistor R. The input signal A to the controller120 is obtained by averaging the difference between the output (blacklevel) from the blanking level (black level) detector 140A and theluminance signal by using the RC integrator 140B.

The input signal B to the controller 120 is obtained by decoding thecontrol signal from the infrared remote controller by using the decoder121. This signal can be set to an arbitrary value by the infrared remotecontroller. As illustrated in FIG. 18 the input signal B changes themagnitude of the effect that the input signal A has on the aperturestops. When the value of the input signal B is sufficiently small, theangle of collection of each aperture stop is held minimized regardlessof the input signal A. When the value of the input signal B issufficiently large, the angle of collection of each aperture stop isheld maximized regardless of the input signal A. To fix the angle ofcollection at a specific value, the input signal A is held constant atan intermediate signal fixed value, FIG. 18, by using a switch (notshown) of the controller 120.

The driver 107 is characterized by receiving, as its one input, anoutput signal from the smoothing circuit 140B and correcting the voltagefor driving the scattering type liquid crystal panel. In thiscorrection, the operation state of the controller 120 is detected on thebasis of the decoded signal from the decoder 121, and the driving signalis corrected in synchronism with the controller such that a change inthe average intensity of the signal is decreased. Therefore, if aprojected image which is originally dark is further darkened with adecrease in the angle of collection, the driving signal is correctedsuch that the luminance of the scattering type liquid crystal panel isincreased. Consequently, a variation in the luminance is reduced in theprojected image finally obtained.

The display apparatus with the arrangement illustrated in FIG. 6A wasoperated to perform a display in a dark room. The result was that thecontrast was 70:1 for a angle of collection of 1.1×10⁻³ sr and 18:1 fora angle of collection of 8.6×10⁻³ sr, because of unsatisfactorycharacteristics of the polymer dispersion type liquid crystal. Thequantity of light in a white image display was 18 lm for an angle ofcollection of 1.1×10⁻³ sr and 75 lm for an angle of collection of8.6×10⁻³ sr. When the aperture stops were fixed, it was necessary to usethe aperture stops at the minimum angle of collection of 1.1×10⁻³ sr inorder to obtain a sufficient contrast. When the aperture stops werevariable as in this embodiment, in contrast, the impression of thedisplay was dramatically improved since the entire luminance could beraised in bright scenes. Especially when the display operation was doneby using video software recording pictures of constellations and thelunar world, the blackness of the background was enhanced in the scenesof constellations, resulting in a very good display entirely differentfrom that obtained with the fixed aperture stops. In addition, to checkthe relationship between the average luminance change and the apertureadjustment speed, the time constant of the aperture adjustment waschanged from 0.5 to about φ1 sec. As a result, the displaycharacteristics were improved with little unnaturalness.

The display operation was also done by using a screen with a reflectiongain of 13 times in a 500-lux room. Consequently, the brightness of thescreen resulting from the internal light in the room was ratherdisturbing. The best impression was obtained when the angle ofcollection was set to nearly a maximum. This setting is worse thandecreasing the angle of collection, since an insufficient contrastresults if the room is sufficiently dark. It was confirmed that the useof the display apparatus shown in FIG. 6A made it possible to performthe display operation by optimizing the display characteristics even ifthe luminance of the use environment changed.

In a bright environment, the sense of a human recognizes a portion whichis dark by contrast with the brightness as black. Therefore, therequirement for a black display in bright scenes is not very strict. Inthis case a sufficient brightness of white portions is more importantthan the contrast.

Conversely, in dark scenes the sense of a human becomes sensitive to thedarkness so that light black and dark black are clearly distinguished.The brightness of white portions is emphasized by contrast with blackportions surrounding the white portions, and so an absolute luminance ofwhite portions is not so important. In this case it is required that thecontrast be high enough to display sufficiently dark black.

The projection display apparatus shown in FIG. 6A can change the displaycharacteristics to satisfy the sense of a human and also can obtain adisplay characteristic that has never been obtained by conventionalapparatuses. That is, it is possible to improve an essential brightnessat a sufficiently high contrast.

This projection display apparatus is put into practical use in variousenvironments depending on, e.g., the period of time or the location. Inparticular, since external light (illumination of a room or light from awindow) acts on the screen to determine its luminance, black images arereadily affected by the external light. That is, if a portionsurrounding the screen is too bright, the contrast of an image displayedon the screen decreases even when the output light from the displayapparatus has a high contrast. In these circumstances, therefore,display is performed by giving priority to the luminance rather than thecontrast. In a sufficiently dark room or the like place, a display isperformed such that black is shown with a sufficient darkness even ifwhite is somewhat darkened.

In the projection display apparatus shown in FIG. 6A, it is possible tofreely choose which of a black display or a white display is givenpriority in accordance with various conditions of the surroundingenvironment. In addition, this display apparatus consumes less powerregardless of the above-mentioned superior display performance.

Note that in the display apparatus shown in FIG. 6A, the reflectionscreen SC is placed on the front side to display images. However, it isalso possible to use a transmission screen placed on the rear side todisplay images.

In addition, in the display apparatus, shown in FIG. 6A is so designedas to use the transmitted light from the scattering type liquid crystalpanel, which is provided as a light-modulating device, as the modulatedlight. However, the reflected light from the scattering type liquidcrystal panel also can be used. This scattering type liquid crystalpanel can have a fine-particle dispersion type liquid crystal layer,instead of the polymer dispersion type liquid crystal layer, as thelight-modulating layer. Also, the scattering type liquid crystal panelcan be replaced with a light-modulating device such as a DMD, TN typeliquid crystal device and slant field effect liquid crystal diffractiongrating.

A projection display apparatus according to the first aspect of thepresent invention will be described below with reference to FIG. 6B.

Referring to FIG. 6B, this projection display apparatus comprises anoptical system in which a spheroidal mirror 101, a light source lamp102, an electric aperture stop 104, a collimator lens 103, a scatteringtype modulating device 108, a field lens 109, projection lenses 110, andanother electric aperture stop 111 are arranged on the optical axis. Thelight from the lamp 102 enters the collimator lens 103 directly andafter being reflected by the mirror 101. The collimator lens 103 outputsthis incident light as parallel rays to the modulating device 108. Themodulating device 108 has a function of modulating the spatialpropagation direction of incident light in a two-dimensional region andis driven by a modulating device driver 107. The modulating device 108is a liquid crystal panel having, between a pair of transparentelectrode substrates, a liquid crystal layer in which a liquid crystalmaterial is dispersed in a polymer resin. The field lens 109 guides themodulated light from the modulating device 108 to the projection lenses110. The projection lenses 110 project the modulated light onto areflection screen SC. That is, the basic display principle of thisprojection display apparatus is identical with the conventionalprinciple.

In this display apparatus, the electric aperture stop 104 is arrangedbetween the light source lamp 102 and the collimator lens 103, and theelectric aperture stop 111 is arranged within the projection lenses 110.The electric aperture stop 104 stops down the beam of the light from thelight source lamp 102 in order to control the angle range of light raysto be incident on the modulating device 108. The electric aperture stop111 stops down the beam of the modulated light from the modulatingdevice 108 in order to control the angle range of light rays to beprojected onto the screen SC.

Each of the electric aperture stops 104 and 111 has an internal servomotor M which is controlled by an aperture stop driver 123. The aperturesize, i.e., the shape of the aperture of each aperture stop is adjustedby the action of this servo motor. The shape of the aperture can be,e.g., rectangular or circular. More preferably, the aperture shape iscircular, as in FIG. 7A, and a radius r of the aperture is changed bythe servo motor. Alternatively, as illustrated in FIG. 7B, light isinterrupted in the upper and lower portions and/or the right and leftportions of the aperture by the servo motor.

It is more preferable that the electric aperture stops 104 and 111 havea rectangular shape in which light is interrupted in the upper and lowerportions, and right and left portions.

It is most preferable that the electric aperture stops 104 and 111 havean arrangement similar to that of an aperture stop often used in acamera, as depicted in FIGS. 8A to 8C. FIGS. 8A, 8B, and 8C illustratethe states in which the aperture diameter is small, medium, and large,respectively.

In the embodiment shown in FIG. 6B, each of the electric aperture stops104 and 111 consists of five ceramic blades CB with a high heatresistance. Each electric aperture stop functions as a circular variableaperture stop whose angle of collection is changed by altering the size,i.e., the radius r of the circular aperture constituted by thecombination of the ceramic blades CB by using the servo motor M.

This display apparatus further comprises a display controller 121, araster signal generator 122, the aperture stop driver 123, a photosensor128, and a photosensor interface circuit 124. The display controller 121controls the whole display operation. The raster signal generator 122generates a raster signal set at a luminance level designated by thedisplay controller 121. The aperture stop driver 123 drives the electricaperture stops 104 and 111 so that each aperture stop has a angle ofcollection, i.e., an aperture radius r designated by the displaycontroller 121. The photosensor 128 detects the luminance of the screenSC to generate an analog voltage signal corresponding to the detectedluminance. The photosensor interface circuit 124 converts the voltagesignal from the photosensor 128 into a digital signal and applies thedigital signal to the display controller 121. The display controller 121is connected to the raster signal generator 122, the aperture stopdriver 123, and the photosensor interface circuit 124 via a data bus125. The photosensor 128 is connected to the photosensor interfacecircuit 124. The aperture stop driver 123 is connected to the electricaperture stops 104 and 111. The modulating device driver 107 isconnected to the modulating device 108.

This display apparatus also includes switches SW1 and SW3 which arecontrolled in accordance with the state of the raster signal generator122, and a push switch SW2 for instructing the aperture adjustment. Theswitch SW1 has a first contact connected to a video input terminal 131to which a picture signal is supplied, a second contact connected to theraster signal output terminal of the raster signal generator 122, and acommon contact connected to the modulating device driver 107. The switchSW1 also has a control terminal connected to the status output terminalof the raster signal generator 122. The switch SW2 is connected betweena pair of switch connecting terminals of the display controller 121. Theswitch SW3 has a first contact connected to one end of a variableresistor 127, a second contact connected to one end of a fixed resistor126, and a common contact connected to one resistor connecting terminalof the aperture stop driver 123. The switch SW3 also has a controlterminal connected to the status output terminal of the raster signalgenerator 122. The other end of each of the fixed resistor 126 and thevariable resistor 127 is connected to the other resistor connectingterminal of the aperture stop driver 123.

The display controller 121 sets the aperture adjustment mode upondetecting that the switch SW2 is pushed. The raster signal generator 122is normally kept in an inoperative state and set in an operative statein this aperture adjustment mode. The switch SW1 connects the commoncontact to the first contact under the control of a status signal whichis supplied when the raster signal generator 122 is in the inoperativestate. The switch SW1 connects the common contact to the second contactunder the control of a status signal which is supplied when the rastersignal generator 122 is in the operative state. That is, the switch SW1supplies a picture signal to the modulating device driver 107, and theswitch SW3 connects the variable resistor 127 to the aperture stopdriver 123. When the raster signal generator 122 is rendered operativeupon setting of the aperture adjustment mode, the switch SW1 supplies araster signal to the modulating device driver 107, and the switch SW2connects the fixed resistor 126 to the aperture stop driver 123.

When connected to the fixed resistor 126, the aperture stop driver 123adjusts the electric aperture stops 104 and 111 to have a angle ofcollection, i.e., an aperture radius designated by the displaycontroller 121. The variable resistor 127 is provided to further correctthe angle of collection thus set and is operated manually. When theaperture stop driver 123 is connected to the variable resistor 127 andthe variable resistor 127 is operated, the aperture stop driver 123corrects the value of the angle of collection in either the positive ornegative direction in accordance with the resistance of the variableresistor 127.

The photosensor 128 consists of, e.g., a photodiode and a collimatorlens. The luminance of the screen SC is measured on the basis of asignal supplied from the photosensor 128 to the display controller 121via the photosensor interface circuit 124.

The display controller 121 includes a microprocessor MP for performingvarious data processing and a memory SM for storing control programs ofthe microprocessor MP and various data. The control programs include aprocessing routine for determining an optimum angle of collection in theaperture control mode.

The operation of this display apparatus will be described below withreference to the flow charts shown in FIGS. 9 and 10.

Referring to FIG. 9, the display controller 121 performs the displaycontrol processing, FIG. 2, by executing control programs upon turningon of a power supply. When this display control processing is started,the display controller 121 checks in step S201 whether the push switchSW2 is pushed. If it is detected that the push switch SW2 is pushed, thedisplay controller 121 sets the aperture adjustment mode to make theraster signal generator 122 operative in step S202. The displaycontroller 121 designates the raster signal generator 122 to maximizethe luminance level of the raster signal. The switch SW1 supplies theraster signal generated by the raster signal generator 122 to themodulating device driver 107. The switch SW3 connects the fixed resistor126 to the aperture stop driver 123. The modulating device driver 107drives the modulating device 108 such that the light transmittance(modulation degree) reaches a maximum in correspondence with the rastersignal. Consequently, the modulating device 108 outputs the brightestmodulated light by which a white image is displayed on the entire screenSC. Thereafter, in step S203 the display controller 121 performsmeasurement for obtaining the luminances of the screen SC with respectto various angle of collections of the aperture stops.

FIG. 10 shows details of this measurement. In step S251, the displaycontroller 121 waits for a predetermined time of about 0.5 second whichis determined in consideration of the response time required to actuallydrive the modulating device 108. In step S252, the display controller121 designates an angle of collection of 1.0×10⁻³ sr to the aperturestop driver 123 in order to set each aperture stop in the most stoppeddown state. With each aperture stop in the most stopped down state, thedisplay controller 121 measures the luminance of the screen detected bythe photosensor 128 in step S254. Thereafter, the display controller 121increases the angle of collection at a rate of 0.5×10⁻³ sr in step S253.In step S254, the display controller 121 measures the luminance of thescreen detected by the photosensor 128 each time the angle of collectionis increased. If the display controller 121 detects in step S255 thatthe angle of collection has reached a value of 9.5×10⁻³ sr by which eachaperture stop is in the fully open state, the display controller 121ends the measurement for the white image display. Thereafter, the flowadvances to step S204 in FIG. 9.

In step S204, the display controller 121 commands the raster signalgenerator 122 to minimize the luminance level of the raster signal instep S202. In this case the switches SW1 and SW2 function in the samemanner as in step S202. The modulating device driver 107 drives themodulating device 108 such that the light transmittance (modulationdegree) is a minimum in correspondence with the raster signal.Consequently, the modulating device 108 outputs the darkest modulatedlight by which a black image is displayed on the entire screen SC.Thereafter, the display controller 121 executes the measurementillustrated in FIG. 10 in step S205 again. When the display controller121 completes the measurement for the black image display, the flowadvances to step S206.

In step S206, the display controller 121 forms a data table indicatingthe relationship between the angle of collection and the contrast. Thisdata table is formed by calculating, as the contrast, the ratio of themeasured value of the luminance obtained in step S203 to the measuredvalue of the luminance obtained in step S205 for the same angle ofcollection. In step S207, the display controller 121 finds an optimumangle of collection by which the contrast is a maximum from the datatable, and designates this optimum angle of collection to the aperturestop driver 123. The aperture stop driver 123 drives the electricaperture stops 104 and 111 to have the optimum angle of collection.Thereafter, the display controller 121 releases the aperture adjustmentmode to set the raster signal generator 122 in an inoperative state, andexecutes step S201 again. After the release of the aperture adjustmentmode, the switch SW1 supplies a picture signal, which is received at thevideo input terminal, to the modulating device driver 107, and theswitch SW3 connects the variable resistor 127 to the aperture stopdriver 123.

In the above embodiment, in response to the push operation of the switchSW2 an optimum angle of collection of the aperture stops by which thecontrast is a maximum is calculated, and the aperture stops are soautomatically adjusted as to have this optimum angle of collection.Since the variable resistor 127 is made usable after this adjustment,the luminance of the displayed image obtained at the optimum angle ofcollection can be further increased or decreased in accordance with theuser's taste.

The results of a display experiment obtained by the modulating deviceusing a polymer dispersion type liquid crystal were as follows. Thecontrast was 70:1 for a angle of collection of 1.0×10⁻³ sr and 18:1 fora angle of collection of 9.5×10⁻³ sr. The quantity of light in the whiteimage display was 18 lm for a angle of collection of 1.0×10⁻³ sr and 75lm for a converting angle of 9.5×10⁻³ sr. The display experiment wasalso conducted while varying the luminance of the room. By pushing theswitch SW2 each time the luminance of the room was changed, the angle ofcollection of the aperture stops 104 and 111 was rapidly adjusted to anoptimum value at which the contrast was a maximum. This allowed theapparatus to readily cope with changes in the use environment.

Another embodiment of the projection display apparatus according to thefirst aspect of the present invention will be described below withreference to FIG. 11.

This display apparatus is constituted by exactly the same hardware asthat of the apparatus illustrated in FIG. 6B except for the contents ofa memory SM provided in a display controller 121. Therefore, the samereference numerals as in the above embodiment denote the same parts inthis embodiment, and a detailed description of the hardware will beomitted. In this embodiment, the memory SM prestores a data tableindicating a standard relationship between various angle of collectionswithin a common variable range of electric aperture stops 104 and 111and the intensities of modulated light from a modulating device whichare output to display white and black images at these angle ofcollections. Control programs are modified to use this table.

That is, the display controller 121 performs display control processingshown in FIG. 11 by executing the control programs upon turning on of apower supply. When this display control processing is started, thedisplay controller 121 checks in step S301 whether a push switch SW2 ispushed. If it is detected that the push switch SW2 is pushed, thedisplay controller 121 sets an aperture adjustment mode to render araster signal generator 122 operative in step S302. The displaycontroller 121 also designates a standard aperture stop angle ofcollection of 2.1×10⁻³ sr to an aperture stop driver 123. In step S303,the display controller 121 commands the raster signal generator 122 tomaximize the luminance level of the raster signal. A switch SW1 suppliesthe raster signal generated by the raster signal generator 122 to amodulating device driver 107. A switch SW3 connects a fixed resistor 126to the aperture stop driver 123. The modulating device driver 107 drivesa modulating device 108 such that the light transmittance (modulationdegree) becomes a maximum in correspondence with the raster signal.Consequently, the modulating device 108 outputs the brightest modulatedlight with which a white image is displayed on an entire screen SC.Thereafter, in step S304 the display controller 121 waits for apredetermined time of about 0.5 second which is determined inconsideration of the response time required to actually drive themodulating device 108. In step S305, with the angle of collection of theaperture stops set at 2.1×10⁻³ sr, the display controller 121 measuresthe luminance detected by a photosensor 128. In step S306, the displaycontroller 121 commands the raster signal generator 122 to minimize theluminance level of the raster signal. In this case the switches SW1 andSW3 function in the same fashion as in step S303. The modulating devicedriver 107 drives the modulating device 108 such that the lighttransmittance (modulation degree) is a minimum in correspondence withthe raster signal. Consequently, the modulating device 108 outputs thedarkest modulated light with which a black image is displayed on theentire screen SC. Thereafter, in step S307 the display controller 121waits for a predetermined time of about 0.5 second which is determinedin consideration of the response time required to actually drive themodulating device 108. In step S308, with the angle of collection of theaperture stops set at 2.1×10⁻³ sr, the display controller 121 measuresthe luminance detected by the photosensor 128.

In step S309, the display controller 121 calculates a projectioncoefficient q and a luminance L₀ of the background contained in anequation L_(ON) =qI_(ON) +L₀ indicating the relationship betweenluminance L_(ON), intensity I_(ON) of the output modulated light fromthe modulating device 108, and the luminance L₀ of the backgroundobtained when the white image is displayed on the screen, and in anequation L_(OFF) =qI_(OFF) +L₀ indicating the relationship betweenluminance L_(OFF), intensity I_(OFF) of the output modulated light fromthe modulating device 108, and the luminance L₀ of the backgroundobtained when the black image is displayed on the screen. That is, themeasured values of L_(ON) and L_(OFF) obtained in steps S305 and S308,together with the intensities I_(ON) and I_(OFF) of the modulated light,are substituted into these equations. In these equations, q is apositive constant, and I_(ON) and I_(OFF) are values obtained from thedata table stored in the memory SM. Thereafter, in step S310 the displaycontroller 121 substitutes q and L₀ calculated in step S309 into the twoequations L_(ON) =qI_(ON) +L₀ and L_(OFF) =qI_(OFF) +L₀ shown in stepS309, and also substitutes the intensities I_(ON) and I_(OFF) of themodulated light, which are obtained from the data table by assuming thatthe angle of collection is altered within the common variable range ofthe aperture stops, thereby calculating a angle of collection at whichthe contrast L_(ON) /L_(OFF) is a maximum. In step S311, the displaycontroller 121 designates this angle of collection to the aperture stopdriver 123. The aperture stop driver 123 drives the aperture stops 104and 111 to have this angle of collection. Thereafter, the displaycontroller 121 releases the aperture adjustment mode to set the rastersignal generator 122 in an inoperative state in step S312, and executesstep S301 again. After the release of the aperture adjustment mode, theswitch SW1 supplies a picture signal, which is input to a video inputterminal, to the modulating device driver 107, and the switch SW3connects a variable resistor 127 to the aperture stop driver 123.

In this second embodiment, as in the first embodiment discussed above,in response to the push operation of the switch SW2 an optimum angle ofcollection of the aperture stops by which the contrast is a maximum iscalculated, and the aperture stops are so automatically adjusted to havethis optimum angle of collection. The variable resistor 127 is renderedusable after this adjustment. Consequently, the luminance of thedisplayed image obtained at the optimum angle of collection can befurther increased or decreased in accordance with the user's taste.

Also, in this embodiment it is possible to optimize the aperture stopangle of collection within a short time period. That is, in the firstembodiment, the aperture stop angle of collection is changed at a rateof 0.5×10⁻³ sr within the variable range while a white or black image isdisplayed on the screen, and it is necessary to measure the luminance ofthe screen each time the angle of collection is altered. In the secondembodiment, however, the number of the screen luminance measurements canbe reduced to 2 by the use of the data table already prepared in thememory SM. As a result, the time required to optimize the angle ofcollection of the aperture stops can be shortened to 5 seconds or less.

In addition, in this second embodiment the luminance L is measured byusing the two exit light components, i.e., the modulated light with thehighest light intensity I_(ON) and the modulated light with the lowestlight intensity I_(OFF) obtained by the standard angle of collection, inorder to estimate q and L₀ in the equation L=qI+L₀. However, some othermodulated states can also be used without departing from the gist of thepresent invention.

As an example, q and L₀ can be calculated by substituting, into thefollowing equations, light intensity I_(AMAX) of modulated lightobtained when the angle of collection is a maximum and light intensityI_(AMIN) of modulated light obtained when the angle of collection is aminimum at a modulation degree of the modulating device set at apredetermined intermediate level, and luminances L_(AMAX) and L_(AMIN)measured using these modulated light components as exit lightcomponents:

L_(AMAX) =qI_(AMAX) +L₀

L_(AMIN) =qI_(AMIN) +L₀

The point is to project modulated light components with at least twodifferent light intensities onto the screen SC and measure the resultingluminances of the screen SC in order to calculate q and L₀. Note thatsince the accuracy of q and L₀ improves as the intensity differencebetween these modulated light components increases, it is preferable tomaximize the intensity of one modulated light and minimize that of theother.

It is also possible to project n (=3 or more) different modulated lightcomponents instead of projecting two modulated light components of eachdifferent intensity. The luminance obtained by projection of thesemodulated light components is represented by the following equation:

    L.sub.i =qI.sub.i +L.sub.0 (i=1, 2, . . . 3)

In this case q and L₀ can be calculated by using, e.g., a method ofleast squares. When q and L₀ are obtained, it is possible to determinean optimum aperture condition corresponding to the ratio of q and L₀thus calculated, from a one-dimensional data table indicating optimumaperture conditions for various q/L₀ ratios.

In each of the above embodiments, the reflection screen SC is placed onthe front side to display images. However, it is also possible to use atransmission screen placed on the rear side to display images.

In addition, in each embodiment the display apparatus is so designed asto use the transmitted light from the modulating device as the modulatedlight. However, the reflected light from the modulating device can alsobe used. Furthermore, it is possible to use, e.g., a fine-particledispersion type liquid crystal, a DMD, and slant field effect liquidcrystal diffraction grating instead of the polymer dispersion typeliquid crystal of the modulating device.

According to the first aspect of the present invention, the luminance ofthe screen which reflects the angle distribution of an incident lightbeam to the modulating device is used in the aperture adjustment bywhich the contrast of a displayed image is maximized. This makes itpossible to obtain displayed images easier to see under any environmentin which the screen is placed.

A projection display apparatus according to the second aspect of thepresent invention will be described below with reference to FIG. 12.

FIG. 12 shows the arrangement of this projection display apparatus. Theprojection display apparatus comprises an optical system having aspheroidal mirror 101, a light source lamp 102, a collimator lens 103,an electric aperture stop 104, a modulating device 108, a field lens109, projection lenses 110, and another electric aperture stop 111. Thelight from the lamp 102 enters the collimator lens 103 directly andafter being reflected by the mirror 101. The collimator lens 103 outputsthis incident light as parallel rays to the modulating device 108. Themodulating device 108 includes a liquid crystal layer, in which a liquidcrystal material is dispersed in a polymer resin, as a light-modulatinglayer between a pair of transparent electrode substrates. The modulatingdevice 108 is driven by a modulating device driver 107 as alight-modulating device which modulates the spatial propagationdirection of light by using this light-modulating layer. The modulatedlight from the modulating device 108 is incident on the projectionlenses 110 through the field lens 109. The projection lenses 110 projectthe modulated light onto a reflection screen SC. That is, the basicarrangement of this projection display apparatus is identical with thatof the projection display apparatus illustrated in FIG. 6B.

As with the apparatus in FIG. 6B, this display apparatus includes thetwo electric aperture stops 104 and 111. The electric aperture stop 104narrows the bundle of rays entering the collimator lens 103. Theelectric aperture stop 111 narrows the bundle of rays projected from theprojection lenses 110. A controller 120 operates the aperture stops 104and 111 on the basis of an input signal A from a luminance signalsmoothing circuit 140, an input signal B from a decoder 121, and asignal C from a photosensor interface circuit 124, thereby controllingthe distribution of the incident light beam to the modulating device 108and the exit beam angle range which contributes to a display. Thedecoder 121 receives a control signal transmitted from an externalinfrared remote controller and decodes the received signal to obtain thesignal B.

FIG. 13 shows the relationship between the input signals A and B to thecontroller 120 and the angle of collection of the aperture stops. Thestate of each aperture stop is represented by the angle at which theexit light beam from the scattering type liquid crystal panel 108 passesthrough the aperture stop, i.e., the angle of collection. The angle ofcollection of the aperture stop 111 is so set as to be variable withinthe range from 8.6×10⁻³ to 1.1×10⁻³ sr. The aperture stop 104 is socontrolled that a light beam within the same angle range is incident onthe scattering type liquid crystal panel 108.

The input signal A to the controller 120 indicates the time averageintensity of a luminance signal contained in a picture signal. The inputsignal A is generated by the luminance signal smoothing circuit 140. Asin FIG. 12, this luminance signal smoothing circuit 140 is composed of aluminance signal blanking level (black level) detector 140A and an RCintegrator 140B. A time constant RC of the integrator 140B can bealtered by adjusting a resistor R. The input signal A to the controller120 is obtained by averaging the difference between the output (blacklevel) from the blanking level (black level) detector 140A and theluminance signal by using the RC integrator 140B.

The input signal B to the controller 120 is obtained by decoding thecontrol signal from the infrared remote controller by using the decoder121. This signal can be set to an arbitrary value by the infrared remotecontroller. As illustrated in FIG. 13, the input signal B changes themagnitude of the effect that the input signal A has on the aperturestops. When the value of the input signal B is sufficiently small, theangle of collection of each aperture stop is held minimized regardlessof the input signal A. When the value of the input signal B issufficiently large, the angle of collection of each aperture stop isheld maximized regardless of the input signal A. To fix the angle ofcollection at a specific value, the input signal A is held constant atan intermediate signal fixed value, FIG. 13, by using a switch (notshown) of the controller 120.

The driver 107 is characterized by receiving, as its one input, anoutput signal from the smoothing circuit 140B and correcting the voltagefor driving the scattering type liquid crystal panel. In thiscorrection, the operation state of the controller 120 is detected on thebasis of the decoded signal from the decoder 121, and the driving signalis corrected in synchronism with the controller such that a change inthe average intensity of the signal is decreased. Therefore, if aprojected image which is originally dark is further darkened with adecrease in the angle of collection, the driving signal is correctedsuch that the luminance of the scattering type liquid crystal panel isincreased. Consequently, a variation in the luminance is reduced in theprojected image finally obtained.

After the input signal A is held constant at the intermediate signalfixed value, FIG. 13, as discussed above, the aperture stops arecontrolled on the basis of the signal C from the photosensor interfacecircuit 124.

The average picture level (APL) detector 140 of the display apparatus inFIG. 12 will be described in detail below with reference to FIGS. 14 to17.

In the average picture level (APL) detector 140 illustrated in FIG. 14,an input positive picture signal from an input terminal 151 is appliedto the collector of a transistor 174 via a coupling capacitor 171. Thecollector of the transistor 174 is biased with a voltage obtained bydividing a power supply voltage V_(CC), which is supplied to a powersupply terminal 161, by resistors 172 and 173.

The base of the transistor 174 is applied with a gate pulse from aterminal 162 via a resistor 175. The transistor 174 detects an averagepicture level (APL) during the period of the gate pulse. This period ofdetection is, e.g., the blanking period of a picture signal.

The detected APL is passed through a buffer transistor 176 andpeak-rectified by a time constant circuit consisting of a rectifierdiode 177, a resistor 178, and a capacitor 179. The rectified signal ispassed through a buffer amplifier consisting of a transistor 180 and aresistor 181, and through a resistor 182, resulting in the signalsexplained below.

FIG. 15 shows the collector input to the transistor 174, the emitteroutput from the buffer transistor 176, and the emitter output from thebuffer transistor 180 for each of a picture signal with a low APL and apicture signal with a high APL. It is evident from FIG. 15 that theemitter output from the buffer transistor 180 is low when the APL is lowand high when the APL is high. The voltage change is obtained via thetransistor 182.

FIG. 16 is a circuit diagram showing another example of the APLdetector. Assuming, for example, that a picture signal which is clampedat the leading edge of a sync signal is applied to a terminal 151. Atransistor 190 is connected to a power supply terminal 161 via aparallel circuit consisting of a resistor 191 and a capacitor 192. Theemitter of the transistor 190 is connected to the reference potentialpoint via a resistor 193 and a Zener diode 194.

The collector of the transistor 190 is connected to the base of a PNPtransistor 195. The emitter of the PNP transistor 195 is connected to apower supply terminal 162 via a parallel circuit consisting of aresistor 196 and a capacitor 197, and is also connected to the base ofthe transistor 180. The transistor 180 and the resistor 182 are similarto those shown in FIG. 15.

In the circuit with the configuration illustrated in FIG. 16, when theclamped picture signal is applied to the base of the transistor 190, asignal with a level equal to or higher than a threshold value set by theZener diode 194 is repeatedly amplified and output from the collector ofthe transistor 190. This signal is rectified by the base-emitter path ofthe transistor 195 and extracted via the transistor 180.

By the emitter output from the transistor 180, the reference voltage atthe input terminal (-) of an error amplifier 150 changes. The resultsare the collector input to the transistor 190 and the emitter outputfrom the buffer transistor 195 as illustrated in FIG. 17. Note that inFIG. 17, each hatched portion represents the portion amplified by thetransistor 195. A portion below this threshold value is rectifiedthrough the base-emitter path of the transistor 180.

The display apparatus with the arrangement illustrated in FIG. 12 wasoperated to perform a display in a dark room. The result was that thecontrast was 70:1 for a angle of collection of 1.1×10⁻³ sr and 18:1 fora angle of collection of 8.6×10⁻³ sr, because of unsatisfactorycharacteristics of the polymer dispersion type liquid crystal. Thequantity of light in a white image display was 18 lm for a angle ofcollection of 1.1×10⁻³ sr and 75 lm for a angle of collection of8.6×10⁻³ sr. When the aperture stops were fixed, it was necessary to usethe aperture stops at the minimum angle of collection of 1.1×10⁻³ sr inorder to obtain a sufficient contrast. When the aperture stops werevariable as in this embodiment, in contrast, the impression of thedisplay was dramatically improved since the entire luminance could beraised in bright scenes. Especially when the display operation was doneby using video software recording pictures of constellations and thelunar world, the blackness of the background was enhanced in the scenesof constellations, resulting in a very good display entirely differentfrom that obtained with the fixed aperture stops. In addition, to checkthe relationship between the average luminance change and the apertureadjustment speed, the time constant of the aperture adjustment waschanged from 0.5 to about 1 sec. As a result, the displaycharacte-ristics were improved with little unnaturalness.

The display operation was also done by using a screen with a reflectiongain of 13 times in a 500-lux room. Consequently, the brightness of thescreen resulting from the internal light in the room was ratherdisturbing. The best impression was obtained when the angle ofcollection was set to nearly a maximum. This setting is worse thandecreasing the angle of collection, since an insufficient contrastresults if the room is sufficiently dark. It was confirmed that the useof the display apparatus of this embodiment made it possible to performthe display operation by optimizing the display characteristics even ifthe luminance of the use environment changed.

In a bright environment, the sense of a human recognizes a portion whichis dark by contrast with the brightness as black. Therefore, therequirement for a black display in bright scenes is not very strict. Inthis case a sufficient brightness of white portions is more importantthan the contrast.

Conversely, in dark scenes the sense of a human becomes sensitive to thedarkness so that light black and dark black are clearly distinguished.The brightness of white portions is emphasized by contrast with blackportions surrounding the white portions, and so an absolute luminance ofwhite portions is not so important. In this case it is required that thecontrast be high enough to display sufficiently dark black.

The projection display apparatus of this embodiment can change thedisplay characteristics to satisfy the sense of a human and also canobtain a display characteristic that has never been obtained byconventional apparatuses. That is, it is possible to improve anessential brightness at a sufficiently high contrast.

This projection display apparatus is put into practical use in variousenvironments depending on, e.g., the period of time or the location. Inparticular, since external light (illumination of a room or light from awindow) acts on the screen to determine its luminance, black images arereadily affected by the external light. That is, if a portionsurrounding the screen is too bright, the contrast of an image displayedon the screen decreases even when the output light from the displayapparatus has a high contrast. In these circumstances, therefore,display is performed by giving priority to the luminance rather than thecontrast. In a sufficiently dark room or the like place, a display isperformed such that black is shown with a sufficient darkness even ifwhite is somewhat darkened.

In the projection display apparatus of this embodiment, it is possibleto freely choose which of a black display or a white display is givenpriority in accordance with various conditions of the surroundingenvironment. In addition, this display apparatus consumes less powerregardless of the above-mentioned superior display performance.

Note that in this embodiment, the reflection screen SC is placed on thefront side to display images. However, it is also possible to use atransmission screen placed on the rear side to display images.

In addition, in this embodiment the display apparatus is so designed asto use the transmitted light from the scattering type liquid crystalpanel, which is provided as a light-modulating device, as the modulatedlight. However, the reflected light from the scattering type liquidcrystal panel also can be used. This scattering type liquid crystalpanel can have a fine-particle dispersion type liquid crystal layer,instead of the polymer dispersion type liquid crystal layer, as thelight-modulating layer. Also, the scattering type liquid crystal panelcan be replaced with a light-modulating device such as a DMD.

The display apparatus according to this embodiment can display imagesmore brightly in bright scenes and more darkly in dark scenes, therebyimproving the effective contrast and luminance. This is so because, asillustrated in FIG. 18, the display characteristics change in accordancewith the state of the aperture stop. Referring to FIG. 18, the state ofthe aperture stop is indicated by the angle at which the exit light beamfrom the light-modulating device passes through the aperture stop, i.e.,the angle of collection. When the angle of collection is small, thedisplay is darkened as a whole. Since the display is particularlydarkened in the case of a black display, the contrast improves. When theangle of collection is increased, not only a white display but a blackdisplay is brightened, resulting in a decreased contrast. Altering theangle of collection in accordance with the luminance of a display makesit possible to use the maximum luminance and the minimum luminance inthe display.

In a bright environment, display characteristics attaching importance tothe luminance can be obtained. In a dark environment, on the other hand,display characteristics with a high contrast attaching importance to theblack level can be obtained. That is, as illustrated in FIG. 19, whenthe luminance of the display screen is constantly held at a fixed valueor higher due to the external brightness, the black level does notdecrease to be lower than that value. In this case it makes nodifference whether the output light beam from the display apparatus isdecreased. Therefore, in this case the overall display characteristicsincluding the environment can be improved by performing a displayattaching importance to the luminance by opening the aperture stops.

According to the second aspect as has been discussed above, it ispossible to properly control the contrast and the luminance of imagesdisplayed on the screen.

The third aspect of the present invention will be described below.

In the display apparatuses according to the first and second aspects orin conventional display apparatuses, when the angle of collection of theaperture stops is adjusted, no desired transmitted light intensity canbe obtained any longer at a driving voltage generated in correspondencewith a luminance signal voltage. That is, the gradation of a displayedimage is not accurately reproduced. In addition, when the modulatingdevice displays a color image consisting of color components of red,green, and blue, the gradation balance between these color componentschanges depending on the angle of collection, resulting in a change inthe tone of the displayed image.

Also, in the projection display apparatuses described above, the displaystate can be optimized under various environments. However, the angledistribution of light rays to be incident on the liquid crystal panelwhen a display is performed by giving priority to the luminance isdifferent from that when a display is performed by giving priority tothe contrast. The result is a reduction in the contrast or a degradationin the display quality. It is found that this degradation in the displayquality is caused by variations in an optical leakage current from aswitching device of the liquid crystal panel, which is due to variationsin light flux incident on the liquid crystal panel.

The third aspect of the present invention has been made in considerationof the above technical problems and provides a projection displayapparatus capable of accurately reproducing the gradation of an image,projecting good images in various environments, and projectinghigh-quality images even when used in a bright environment.

There will be described a dependence of the driving voltage-modulatedlight intensity characteristic of the modulating device on the angle ofcollection controlled by the aperture stop. According to studies made bythe present inventors, the driving voltage-modulated light intensitycharacteristic of the modulating device is shown in FIG. 20. In FIG. 20,a characteristic curve Y1 represents the relationship between thedriving voltage and the transmitted light (modulated light) intensity ofthe modulating device obtained when the angle of collection takes astandard value, 5°. This characteristic curve due to variations in theangle distribution of incident rays on the liquid crystal panel.

It may be possible to suppress the optical leakage current bysufficiently shielding the switching device from light. When theaperture efficiency of the liquid crystal panel is taken intoconsideration, however, it is difficult to completely shield theswitching device from light particularly in a projection displayapparatus which projects images in an enlarged scale, since theeffective use of light from a light source is essential in an apparatusof this sort.

The third aspect of the present invention has been made in considerationof the above technical problems and provides a projection displayapparatus capable of accurately reproducing the gradation of an image,projecting good images in various environments, and projectinghigh-quality images with no degradation in the contrast even when usedin a bright environment.

The present inventors have made extensive studies and found that achange in the driving voltage-modulated light intensity characteristicof the modulating device is dependent on the angle of collection. InFIG. 20, a characteristic curve Y1 represents the relationship betweenthe driving voltage and the transmitted light (modulated light)intensity of the modulating device obtained when the angle of collectiontakes a standard value, 5°. This characteristic curve Y1 shifts asindicated by a curve Y2 when the aperture stop is widened, and shifts asindicated by a curve Y3 when the aperture stop is stopped down. Thisconsequently changes the transmitted light intensity for the samedriving voltage. As an example, when the characteristic curve Y1 of themodulating device shifts to the curve Y2, the transmittance of themodulating device is set at a value higher than 0.6 which is supposed tobe set for a luminance signal voltage of 0.6 V, as shown in FIG. 21.

This dependence of the driving voltage-modulated light intensitycharacteristic on the aperture stop is considered to be caused by thefollowing reason. That is, as illustrated in FIG. 22, the distributionof an incident light beam to the modulating device is equal to thedistribution of an exit light beam which is determined by the angle ofcollection of the aperture stop. As shown in FIG. 23, the modulatingdevice has a property of scattering light. When the angle of collectionof the aperture stop is changed as indicated by A, B, and C in FIG. 22,the exit light beam is distributed as shown in FIGS. 24A, 24B, and 24Cfor the angle of collections A, B, and C, respectively.

Each of these exit light beams, however, is intercepted by the aperturestop on the exit side except in a portion within the same constant anglerange as the distribution of the incident light beam. In FIGS. 24A to24C, only a hatched portion is effective as the exit light beam whichcontributes to a display. As is apparent from FIGS. 24A to 24C, theratio of the effective portion to the light-intercepted portionincreases as the angle of collection widens.

The relative transmittance of the modulating device is the ratio betweenthe quantity of the exit light beam passing through the aperture stop onthe exit side when no incident light beam is scattered by the modulatingdevice and the quantity of the exit light beam passing through theaperture stop on the exit side when the incident light beam is scatteredby the scattering property of the modulating device. When the modulatingdevice has the scattering nature as in FIG. 23, transmittancescorresponding to the aperture amounts, by which the incident and exitlight beams as indicated by A, B, and C in FIG. 22 are obtained, are asshown in FIGS. 24A, 24B, and 24C, respectively. That is, widening theangle of collection increases the ratio of the exit light beamtransmitting through the aperture stop on the exit side to the incidentlight beam.

In the display apparatus according to the third aspect of the presentinvention, a change in the driving voltage-modulated light intensitycharacteristic of the modulating device is compensated for by acompensator which is interlocked with an aperture controller.Consequently, gamma correction can be properly performed even when theluminance of an image is altered by adjusting the aperture amount.Therefore, it is possible to prevent a poor gradation reproduction andan unnatural color change.

The projection display apparatus according to the third aspect of thepresent invention will be described below with reference to theaccompanying drawings.

FIG. 25 shows the overall arrangement of this projection displayapparatus. This projection display apparatus has an optical system inwhich a spheroidal mirror 101, a light source lamp 102, an electricaperture stop 104, a collimator lens 103, a scattering type modulatingdevice 108, a field lens 109, projection lenses 110, and anotherelectric aperture stop 111 are arranged on the optical axis. The lightfrom the lamp 102 is incident on the collimator lens 103 directly andafter being reflected by the mirror 101. The collimator lens 103 outputsthis incident light as parallel rays to the modulating device 108. Themodulating device 108 modulates the spatial propagation direction of theincident light in a two-dimensional region. The modulating device 108 isdriven by a driver constituted by a gamma-correcting circuit 210 and apolarity inverting/noninverting amplifier 211. The modulating device 108is a polymer dispersion type liquid crystal panel in which a liquidcrystal layer formed by dispersing a liquid crystal material in apolymer resin is held between a pair of transparent electrodesubstrates. A plurality of color pixel groups are arranged in a matrixmanner in this liquid crystal panel. Each pixel group consists of red,green, and blue pixels. The field lens 109 guides the modulated lightfrom the modulating device 108 to the projection lenses 110. Theprojection lenses 110 project the modulated light onto a reflectionscreen. That is, the basic display principle of this projection displayapparatus is identical with that of conventional apparatuses.

In this display apparatus, the electric aperture stop 104 is arrangedbetween the light source lamp 102 and the collimator lens 103, and theelectric aperture stop 111 is arranged within the projection lenses 110.The electric aperture stop 104 stops down the beam of the light from thelight source lamp 102 in order to control the angle range of light raysto be incident on the modulating device 108. The electric aperture stop111 stops down the beam of the modulated light from the modulatingdevice 108 in order to control the angle range of light rays to beprojected onto a screen SC. Each of the electric aperture stops 104 and111 consists of five ceramic blades with a high heat resistance and hasa servo motor M. Each aperture stop functions as a circular variableaperture stop whose angle of collection is changed by changing the size,i.e., the radius of the circular aperture constituted by the combinationof the ceramic blades by using the servo motor.

This display apparatus is supplied with three luminance signals R, G,and B obtained by separating a color picture signal into colorcomponents of red, green, and blue. Each luminance signal sequentiallydesignates, in units of field periods, the luminance of a pixel of thecorresponding color provided in the modulating device 108. To optimizethe contrast and the luminance of a displayed image, the displayapparatus further includes an aperture controller 212 for performing acommon adjustment for the angle of collections, i.e., the aperture radiiof the electric aperture stops 104 and 111 on the basis of the luminancesignals R, G, and B. This aperture controller 212 supplies angle ofcollection data D, which represents the adjusted aperture stop angle ofcollection, to the gamma-correcting circuit 210. On the basis of theangle of collection data D, the gamma-correcting circuit 210 performsgamma correction for the luminance signals R, G, and B and suppliesresulting driving voltages RO, GO, and BO to the polarityinverting/noninverting amplifier 211. The amplifier 211 inverts thepolarity of the driving voltages RO, GO, and BO from positive tonegative or vice versa in, e.g., the horizontal scanning period of thepicture signal, and supplies the inverted driving voltages to themodulating device 108.

FIG. 26 shows details of the configuration of the gamma-correctingcircuit 210. The gamma-correcting circuit 210 has processing channelsCH1, CH2, and CH3 with an identical arrangement in order to performgamma correction for the luminance signals R, G, and B, respectively.Each of these processing channels CH1 to CH3 includes an A/D converter201, a gamma characteristic conversion table 202, a correction table203, an adder 204, and a D/A converter 205. The A/D converter 201converts a luminance signal of the corresponding color into luminancedata I in a digital form which represents the luminance of each pixel.The gamma characteristic conversion table 202 generates referencedriving voltage data VR corresponding to the luminance data I from theA/D converter 201. The correction table 203 generates correction data.increment.V corresponding to the angle of collection data D from theaperture controller 212. The adder 204 adds the correction data a V tothe reference voltage data VR. The D/A converter 205 converts theaddition result into a driving voltage signal in an analog form.

The characteristic conversion table 202 is a ROM storing a plurality ofreference driving voltages VR to be selected in accordance with theluminance data I. Each reference driving voltage VR represents thereference driving voltage which is determined by the characteristiccurve Y1, FIG. 20, with respect to the luminance designated by theluminance data I. The correction table 203 is a ROM storing a pluralityof correction data .increment.V to be selected in accordance with theluminance data I. Each correction data .increment.V indicates thecorrection voltage obtained by approximating the difference between acurve, such as the curve Y2 or Y3, FIG. 20, which is obtained for theangle of collection designated by the angle of collection data D, andthe characteristic curve Y1.

The operation of each processing channel will be described below. TheA/D converter 201 converts the luminance signal into the luminance dataI and supplies the data to the gamma characteristic conversion table202. Consequently, one of the reference driving voltage data VR storedin the table 202 is selected in accordance with this luminance data Iand supplied to the adder 204 as a result of the gamma correction basedon the characteristic curve Y1 obtained by a standard angle ofcollection, 5°. Meanwhile, one of the correction data .increment.Vstored in the correction table 203 is selected in accordance with theangle of collection data D from the aperture controller 212. Theselected correction data .increment.V is supplied to the adder 204 as acorrection voltage approximating the difference between thecharacteristic curve, which is obtained with respect to the angle ofcollection designated by the angle of collection data D, and thecharacteristic curve Y1. The adder 204 adds the reference drivingvoltage data VR to the correction data .increment.V. The addition resultrepresents a driving voltage suitable for the current angle ofcollection. This addition result is converted into a driving voltagesignal by the D/A converter 205.

When the electric aperture stops 104 and 111 are set at the standardangle of collection of 5°, for example, the correction data .increment.Vindicates 0. Consequently, the addition result from the adder 204 isequal to the reference driving voltage data VR. When the angle ofcollection data D changes upon adjustment of the angle of collection,the correction data .increment.V also changes in either the positive ornegative direction from 0. The correction data .increment.V becomesnegative when the aperture stop angle of collection is larger than 5°,and positive when the angle of collection is smaller than 5°. Themodulating device 108 using the polymer dispersion type liquid crystalis a device which decreases the light scattering degree and increasesthe modulated light intensity upon application of a voltage. Therefore,the driving voltage is so corrected as to decrease as the aperture stopangle of collection is increased, and to increase as the angle ofcollection is decreased.

In this display apparatus, the two electric aperture stops 104 and 111are provided, and the essential characteristic of the gamma-correctingcircuit 210 changes in accordance with the adjustment of the angle ofcollections of these aperture stops. As a result, the driving voltage isappropriately corrected in accordance with the aperture stop angle ofcollection. This makes it possible to obtain a luminance designated bythe luminance signal independently of the angle of collection. That is,neither a poor gradation reproduction nor an unnatural color changeoccurs when the aperture stop angle of collection has changed.

In the above embodiment the angle of collections of both the electricaperture stops 104 and 111 are similarly varied. However, it is alsopossible to vary the angle of collection of only one of these aperturestops. In addition, a fine-particle dispersion type liquid crystal, DMD,or a slant field effect liquid crystal diffraction grating can also beused instead of the polymer dispersion type liquid crystal of themodulating device 108. If a modulating device which increases the lightscattering degree and decreases the modulated light intensity uponapplication of a voltage is used, the contents of the correction table203 are altered to correct the driving voltage such that the drivingvoltage decreases as the aperture stop angle of collection increases.

The correction table 203 holds one correction data .increment.V for eachangle of collection in the above embodiment. However, the correctiontable 203 can also hold a plurality of correction data .increment.Vrepresenting the voltage which is the difference between the curve, suchas the curve Y2 or Y3, obtained for each angle of collection and thereference characteristic curve Y1, for each luminance. In this case,these correction data .increment.V are selected and used in accordancewith the combination of the luminance data I and the angle of collectiondata D. With this arrangement a more precise correction can beperformed, although the storage capacity of the correction table 203must be increased.

Furthermore, instead of guiding the transmitted light from themodulating device 108 to the screen SC, the reflected light from themodulating device 108 can be guided to the screen SC. Also, the screenSC is not limited to a reflection type used to project images on thefront side but can be a transmission type used to project images on therear side.

According to the third aspect of the present invention as discussedabove, while the aperture stop angle of collection is optimized on thebasis of the surrounding environment or the state of a displayed image,the gradation of the displayed image can be accurately reproduced.

The fourth aspect of the present invention will be described below.

Even if no light leaks at the switch element of the modulating device108 as described above, it is necessary to control the drivevoltage-to-modulated light intensity characteristic, depending onchanges of the angle of collection caused by the aperture stop.Practically, however, it is difficult to completely shield the switchelement from light, in view of the numerical aperture. Further, sincethe intensity of the incident light flux varies as the aperture of theaperture stop 52 is controlled, it is also necessary to control saidcharacteristic in view of the leakage of light at the switch element.Light leakage, if any, at the switch element, will deteriorate thepixel-potential holding characteristic. Consequently, the drivevoltage-to-modulated light intensity characteristic will shift toward ahigh-voltage side. The resultant light-leakage current is proportionalto the intensity of the light applied to the switch element. Hence, theextent of the shift increases as the intensity of the light increasesbecause of an increased aperture. The display quality is inevitablydegraded. It is desirable that the switch element be optimallyconstructed and incorporate a polysilicon TFT rather than an amorphoussilicon TFT. It is more preferred that a picture signal control means beused to correct the light-leaking characteristic of the modulatingdevice. Furthermore, since the light-leaking characteristic depends onthe wavelength of the incident light, it is desirable that measures betaken to reduce changes in the hue of the image displayed. The hue ofthe image changes as the gradient balance between the color componentschanges when the angle of collection varies. In practice, the drivevoltage-to-modulated light intensity characteristic should better becorrected, in consideration of the light leakage which takes place whenthe aperture size of the aperture stop is controlled.

In a display apparatus using a polymer dispersion type liquid crystal ora fine-particle dispersion type liquid crystal, the display statechanges with the display luminance or with an elapse of time from theinitial stages of the display, or the display quality changes with theambient temperature. The cause of these problems is considered that, inthe polymer dispersion type liquid crystal or the fine-particledispersion type liquid crystal, the applied voltage (V)-lighttransmittance (T) characteristic, the hysteresis characteristic, or theresponse speed, for example, varies in accordance with temperaturechanges, and this variation is large compared to that in a twistednematic (TN) liquid crystal.

In addition, particularly a projection display apparatus has anotherproblem that the display quality is degraded due to a temperaturedifference resulting from a bundle of rays which changes in accordancewith control of the aperture diameters of aperture stops arranged in alight source optical system.

The fourth aspect of the present invention has been made inconsideration of the above technical problems and provides a displayapparatus capable of projecting high-quality images regardless of thesize of an aperture stop for controlling a light beam, the time elapsedfrom an ON operation of a light source, or the ambient temperature, orwhen images are continuously displayed.

More specifically, as discussed above, a light-modulating deviceincluding a polymer dispersion type (PD) liquid crystal layer in which aliquid crystal material is contained in a polymer resin or afine-particle dispersion type liquid crystal layer in which fineparticles are contained in a liquid crystal has, e.g., an appliedvoltage (V)-light transmittance (T) characteristic, a hysteresischaracteristic, or a response speed which largely varies in accordancewith temperature changes.

FIG. 27 shows the temperature dependence of the modulating device, inwhich the voltage (V₅₀) at which the light transmittance is 50% isplotted on the ordinate, and the temperature (T) is plotted on theabscissa. As is apparent from FIG. 27, to obtain a constant displayedimage independently of the temperature (T), it is necessary to raise theapplied voltage to the liquid crystal layer as the temperature (T)rises.

The fourth aspect of the present invention, therefore, comprises apicture signal control means for increasing or decreasing a picturesignal in accordance with a change in the temperature of the modulatingdevice. With this means a constant displayed image can be obtainedindependently of the temperature (T).

FIG. 28 is a view showing a projection liquid crystal display apparatusaccording to the fourth aspect of the present invention.

As illustrated in FIG. 28, this projection liquid crystal displayapparatus 300 is of a 3-panel type, i.e., includes three modulatingdevices, e.g., liquid crystal panels 301-R, 301-G, and 301-B for red(R), green (G), and blue (B), respectively.

A light source optical system of this display apparatus has a metalhalide lamp as a light source 311 and a spheroidal reflector 321. Thereflector 321 converges the light from the light source 311 to have afocal point at a point A on the optical axis between the light source311 and the liquid panels 301-R, 301-G, and 301-R. The light onceconverged at the point A by the reflector 321 is reflected by a coldmirror 331 and is guided as parallel light to the liquid crystal panels301-R, 301-G, and 301-B by a collimator lens 341. A first aperture stopmeans 351 is arranged at the point A as the focal position of thereflector 321. The first aperture stop means 351 has a nearly circularaperture, and the aperture diameter D₁ of the aperture can be varied bya servo motor.

In this display apparatus, the light source 311 is fixed to the centralportion of the reflector 321 to obtain a prescribed positional accuracywith respect to the reflector 321. Consequently, the light beam in thecenter of the light from the light source 311 is decreased in amount. Tocompensate for this, in the light source optical system of this displayapparatus a convex conical lens 361 is arranged between the light source311 and the first aperture stop means 351 at a position near the firstaperture stop means 351. This conical lens 361 guides the light beam,which is diverged and hence is not effectively used in a display, to thecentral portion of the light, thereby preventing a decrease in the lightamount in the center of the light. Note that a concave conical lens canalso be used in place of the convex conical lens 361 as long as the lensguides the light beam, which is diverged and not effectively used, tothe central portion of the light. Note also that a structure asdisclosed in Jpn. Pat. Appln. KOKAI Publication No. 6-175129 can be usedinstead of the lens 361.

A projection optical system includes a focusing lens 501, a secondaperture stop means 503, and a projection lens 505. The focusing lens501 focuses the light modulated by the liquid crystal panels 301-R,301-G, and 301-B at a point B. The second aperture stop means 503 has anaperture for intercepting the scattered light and transmitting thetransmitted light from the liquid crystal panels 301-R, 301-G, and 301-Bat the point B. An aperture diameter D₂ of this aperture can be variedby a servo motor. The projection lens 505 projects the modulated lightpassing through the second aperture stop means 503. As described above,the second aperture stop means 503 intercepts the scattered light andtransmits the transmitted light from the liquid crystal panels 301-R,301-G, and 301-B. Therefore, the display luminance can be increased byincreasing the aperture diameter D₂ of the second aperture stop means503.

The first and second aperture stop means 351 and 503 are electricallyconnected to an aperture control means 721. The aperture control means721 controls the aperture diameters D₁ and D₂ of the first and secondaperture stop means 351 and 503 on the basis of an environmentalluminance signal ES from a photosensor 711 for monitoring the luminanceon the screen. More specifically, the aperture control means 721controls the aperture diameter D₁ of the first aperture stop means 351and the aperture diameter D₂ of the second aperture stop means 503 suchthat their respective angle of collections Ω₁ and Ω₂ are increased asthe environmental luminance on the screen increases. In this displayapparatus, the aperture diameters D₁ and D₂ of the first and secondaperture stop means 351 and 503 are so controlled that the angle ofcollections Ω₁ and Ω₂ are varied within the range from 8.6×10⁻³ sr to1.1×10⁻³ sr.

Note that assuming the distribution angle of the light-source light is±θ, the angle of collection Ω₁ of the first aperture stop means 351 inthis specification is represented by a value obtained by integrating [2π sin θ] for θ from 0 to θ. In addition, assuming that the focal lengthof the collimator lens 341 is f₁, this angle of collection Ω₁ isexpressed as Ω₁ =π(D₁ /2f₂)² as a function of the aperture diameter D₁of the first aperture stop means 351.

Also, assuming that the focal length of the field lens is f₂, the angleof collection Ω₂ of the second aperture stop means 503 in thisspecification is expressed as Ω₂ =π(D₁ /2f₂)² as a function of theaperture diameter D₂ of the second aperture stop means 503.

When factors such as the light use efficiency and the like are takeninto account, these angle of collections Ω₁ and Ω₂ are desirably variedin association with each other to almost agree with each other.

The arrangement of the liquid crystal panels 301-R, 301-G, and 301-Bwill be described below. Of the light from the light source opticalsystem, only green (G) light is reflected by a first dichroic mirror411-D. The reflected green (G) light is guided to the liquid crystalpanel 301-G by a first total reflection mirror 411-A and output throughthe liquid crystal panel 301-G and a green (G) field lens 421-G.

Of the light-source light transmitted through the first dichroic mirror411-D, only red (R) light is reflected by a second dichroic mirror 413-Dand guided to the liquid crystal panel 301-R. The red (R) light passingthrough the liquid crystal panel 301-R and a red (R) field lens 421-R issynthesized with the green (G) light from the liquid crystal panel 301-Gby a first synthesizing mirror 411-M.

The light-source light transmitted through the second dichroic mirror413-D is guided to the liquid crystal panel 301-B. The blue (B) lightpassing through the liquid crystal panel 301-B and a blue (B) field lens421-B is reflected by a second total reflection mirror 413-A andsynthesized with the red (R) light and the green (G) light, which aresynthesized after being transmitted through the liquid crystal panels301-G and 301-R, by a second synthesizing mirror 413-M. The resultinglight is guided to the projection optical system.

The liquid crystal panels 301-R, 301-G, and 301-B will be describedbelow. Since the arrangements of the liquid crystal panels 301-R, 301-G,and 301-B have no large difference except for the driving systems, theliquid crystal panel 301-G for green (G) will be described as arepresentative. In this liquid crystal panel 301-G, 640 display pixelsand 480 display pixels are arranged in the row and the columndirections, respectively, with a pitch of 100 μm.

As illustrated in FIGS. 29 and 30, in the liquid crystal panel 301-G apolymer dispersion type liquid crystal layer 401, in which a nematicliquid crystal having a positive dielectric anisotropy is dispersed in apolymer resin, is held in place between an array substrate 511 and acounter substrate 611 via surface-treated films 591 and 691.

In the array substrate 511, as in FIG. 29, signal lines 521 and scanninglines 531 are arranged to be substantially perpendicular to each otheron a 0.7-mm thick transparent glass substrate 510. Thin-film transistors(to be abbreviated as TFTs hereinafter) 541 are arranged near theintersections of the signal lines 521 and the scanning lines 531. Asillustrated in FIG. 30, each TFT 541 has a reverse-staggered structureusing the scanning line 531 as a gate electrode and also including athin amorphous silicon film 545, a semiconductor protective film 546, adrain electrode 547, and a source electrode 549. The thin amorphoussilicon film 545 is formed as a semiconductor layer on the scanning line531 via a gate insulating film 543. The semiconductor protective film546 is made from silicon nitride and is self-aligned with the scanningline 531 in order to protect the thin amorphous silicon film 545 andsuppress a parasitic capacitance. The drain electrode 547 extends fromthe signal line 521 and electrically connects the thin amorphous siliconfilm 545 to the signal line 521 via a thin n⁺ -type amorphous siliconfilm 548. The source electrode 549 electrically connects a pixelelectrode 551, which is arranged in the region surrounded by the signallines 521 and the scanning lines 531 and made from ITO (Indium TinOxide), to the thin amorphous film 545 via a thin n⁺ -type amorphoussilicon film 550. Also, auxiliary capacitance lines 553 each for formingan auxiliary capacitance (Cs) between the line 553 and the pixelelectrode 551 via the gate insulating film 543 are arrangedsubstantially parallel to the scanning lines 531. In addition, aprotective film 555 is formed on the TFTs 541 and the pixel electrodes551 to constitute the array substrate 511.

The counter substrate 311 is constituted by a 0.7-mm thick transparentglass substrate 310, a matrix-like light-shielding layer 313, aprotective film 317, and a counter electrode 319. The light-shieldinglayer 313 shields portions surrounding the TFTs 541 and the pixelelectrodes 551 on the array substrate 511 from light and is constructedfrom chromium (Cr). The protective film 317 is formed on thelight-shielding layer 313. The counter electrode 319 is formed on theprotective film 317 and constructed from ITO. The liquid crystal panel301-G with this arrangement achieves an aperture efficiency of 40%.

In this display apparatus, as illustrated in FIG. 31, a temperaturesensor 800 is arranged near the display portion of the plane ofincidence of the liquid crystal module. On the basis of a temperaturesignal (TS) from this sensor, an input brightness signal (BS) to adriving voltage supply circuit 731 is controlled. Consequently, videosignals V_(SR), V_(SG), and V_(SB) are respectively supplied to theliquid crystal panels 301-R, 301-G, and 301-B via a polarityinverting/noninverting amplifier 741.

As the temperature sensor, it is possible to use, e.g., μ PC3911(tradename: manufactured by NEC Corp.) In this IC a reference voltage, atemperature sensor, and an operational amplifier with a phasecompensator are integrated in a single chip, so only a few externalcircuit components are required. In addition, this temperature sensor isvery superior in linearity to conventional temperature sensors such asthermistors and can therefore perform temperature measurements with ahigh accuracy. A practical circuit of the sensor is depicted in FIG. 32.

As shown in FIG. 28, the input brightness signal (BS) to the drivingvoltage supply circuit 731 is controlled on the basis of the temperaturesignal (TS) from the temperature sensor. Consequently, the video signals(V_(SR)), (V_(SG)), and (V_(SB)) are supplied to the liquid crystalpanels 301-R, 301-G, and 301-B, respectively, via a polarityinverting/noninverting amplifier 751.

In the driving voltage supply circuit 731, as illustrated in FIG. 33, ananalog-to-digital converter 733 converts the input brightness signal(BS) into a digital signal. A gamma-correcting circuit 734 constitutedby a ROM performs gamma correction for the brightness signal (BS) in thedigital signal form and supplies the gamma-corrected digital brightnesssignal to one input terminal of an adder 735. In addition, on the basisof the temperature signal (TS) from the temperature sensor, atemperature-compensating circuit 736 constituted by a ROM supplies, tothe other input terminal of the adder 735, compensation data forcorrecting the temperature dependence of the light transmittance(T)-voltage (V) characteristic of the liquid crystal panel 301-G. Theadder 735 outputs the sum of the digital brightness signal and thecompensation data to a digital-to-analog converter 737. Thedigital-to-analog converter 737 outputs an analog signal to the polarityinverting/noninverting amplifier 731.

The polarity inverting/noninverting amplifier 731 amplifies the analogsignal to have a signal level required for the liquid crystal panel301-G and converts the signal into a video signal (V_(SG)) which invertsits polarity with respect to the reference voltage at a predeterminedfrequency for, e.g., each field period or each scan period. Theamplifier 731 supplies the video signal (V_(SG)) to the liquid crystalpanel 301-G.

Likewise, the liquid crystal panels 301-R and 301-B are supplied withvideo signals (V_(SR)) and (V_(SB)) corrected on the basis of thetemperature signal (TS) from the temperature sensor provided in theliquid crystal panel 301-G.

More specifically, this gamma-correcting circuit 734 is so designed asto perform gamma correction for the brightness signal (BS) applied onthe basis of the voltage (V)-light transmittance (T) characteristic whenthe internal temperature of the liquid-crystal panel 301-G is 40° C. Forexample, assuming that the reference internal temperature of the liquidcrystal panel 301-G is 40° C., the compensation data from atemperature-compensating circuit 746 increases toward the positive sideas the temperature rises from 40° C. and decreases toward the negativeside as the temperature falls from 40° C.

A rotational number of the cooling fan for cooling the panel iscontrolled corresponding to the panel temperature monitored by thetemperature sensor, and thus, the temperature of the panel can becontrolled. As a result, it is possible to precisely control the picturesignal corresponding to the panel temperature. It is, therefore,desirable to control the cooling fan on the basis of the output of thetemperature sensor. In addition, when the panel temperature elevatesimmediately after switching-on of the light source, it is desirable tostop the rotation of the fan for a determined time, or to suppress therotation of the fan, thus shortening the time reaching the steadyoperation temperature.

In this projection liquid crystal display apparatus 300 as discussedabove, the display state is optimized in accordance with temperaturechanges in the liquid crystal panels 301-R, 301-G, and 301-B. Therefore,proper driving can be done constantly even if the liquid crystal panels301-R, 301-G, and 301-B keep changing their temperatures for a timeperiod of, e.g., 3 to 30 minutes, which is required for the liquidcrystal panels 301-R, 301-G, and 301-B to reach a predeterminedtemperature after the light source 311 is turned on when the projectionliquid crystal display apparatus 300 is used at relatively low roomtemperatures. Consequently, good displayed images free from colorvariations or the like can be obtained.

In particular, in this display apparatus the aperture diameters D₁ andD₂ of the first and second aperture stop means 351 and 503 are varied inaccordance with the luminance of the screen. Therefore, since theintensity of the incident light to the liquid crystal panels 301-R,301-G, and 301-B changes depending on the aperture diameter D₁ of thefirst aperture stop means 351, the degree of the temperature rise in theliquid crystal panels 301-R, 301-G, and 301-B changes accordingly.

That is, when the screen luminance is small, a high contrast, ratherthan a high display luminance, is visually recognized as a gooddisplayed image. Therefore, the aperture diameter D₁ of the firstaperture stop means 351 is decreased. In contrast, when the screenluminance is large, the display luminance is considered to be of moreimportance. Therefore, the aperture diameter D₁ of the first aperturestop means 351 is increased. Consequently, as the screen luminanceincreases, the degree of the temperature rise in the liquid crystalpanels 301-R, 301-G, and 301-B increases. As an example, in thisembodiment the temperature of the liquid crystal panels 301-R, 301-G,and 301-B changes by 3° to 5° C. when the angle of collection Ω₁ of thelight-source light incident on the liquid crystal panels 301-R, 301-G,and 301-B changes from 8.6×10⁻³ sr to 1.1×10⁻³ sr.

In this embodiment, however, the temperature sensor is arranged in theliquid crystal panel 301-G to constantly monitor the temperature tothereby optimize the video signals (V_(SR)), (V_(SG)), and (V_(SB)). Asa consequence, good displayed images can be obtained even if thetemperature of the liquid crystal panels 301-R, 301-G, and 301-B changesdue to an increase or decrease in the angle of collection Ω₁ of thelight-source light when the aperture diameter D₁ of the first aperturestop means 351 is varied.

In this display apparatus, the aperture diameters D₁ and D₂ of the firstand second aperture stop means 351 and 503 are controlled by the firstaperture control means 721 in accordance with the environmentalluminance signal (ES). It is also possible to control the aperturediameters D₁ and D₂ of the first and second aperture stop means 351 and503 on the basis of the video signals (V_(SR)), (V_(SG)), and (V_(SB))supplied to the liquid crystal panels 301-R, 301-G, and 301-B. In thiscase the control is based on the difference between the time averageintensity of the brightness signal (BS) and the blanking level (blacklevel) of the brightness signal. That is, if the difference is small,the aperture diameters D₁ and D₂ are decreased to decrease the angle ofcollections Ω₁ and Ω₂. If the difference is large, the aperturediameters D₁ and D₂ are increased to increase the angle of collectionsΩ₁ and Ω₂. Consequently, good displayed images can be obtained.

The first and second aperture stop means 351 and 503 can also becontrolled in accordance with both the environmental luminance signal(ES) and the brightness signal (BS).

In this display apparatus, the temperature sensor is so arranged as tobe able to directly monitor the temperature of the polymer dispersiontype (PD type) liquid crystal layer 401. However, it is also possible todetect a signal correlated to the temperature of the polymer dispersiontype liquid crystal layer and perform control on the basis of thedetected signal.

Also, in this display apparatus the temperature of the liquid crystalpanel 301-G is constantly monitored, and the display state is optimizedon the basis of the temperature signal (TS). However, instead of usingthe temperature sensor, compensation data can be set such that the videosignals (V_(SR)), (V_(SG)), and (V_(SB)) are gradually decreased eachpredetermined period of time.

In the above display apparatus, the temperature sensor is arranged in anupper portion of the display portion on the incident side. This positioncan be a lower portion or a right or a left portion as long as thetemperature can be measured accurately. In addition, the temperaturesensor can be either in contact with or spaced apart from the liquidcrystal module or can be arranged inside the liquid crystal module.Furthermore, the temperature sensor can be arranged on the light exitside.

As discussed earlier, the angle of collection of the aperture stop onthe light source side desirably agrees with the angle of collection ofthe aperture stop on the projection side, for otherwise the performancedegrades as follows. That is, if the angle of collection of the aperturestop on the light source side is larger than the angle of collection ofthe aperture stop on the projection side, although the maximum luminanceremains the same as when the two angles are in agreement, the contrastdecreases due to an increase in the luminance in a black display.Likewise, if the angle of collection of the aperture stop on the lightsource side is smaller than the angle of collection of the aperture stopon the projection side, the contrast decreases due to an increase in theluminance in a black display, although the maximum luminance remainsunchanged from that in the agreement case. Since the luminance in ablack display increases almost in proportion to the area of the aperturestop, the contrast also decreases nearly proportionally to the area ofthe aperture stop.

Assuming that the area of the aperture stop is S₀ and the area of theaperture stop with a larger angle of collection is S₀ +.increment.S whenthe angle of collection of the aperture stop on the light source side isin completely agreement with the angle of collection of the aperturestop on the projection side, contrast r' when the angle of collectionsare in disagreement is represented by the following equation as afunction of contrast r when the angle of collections are in agreement.

    r'=r{S.sub.0 /(S.sub.0 +.increment.S)}

According to an experiment made by the present inventors, a decrease inthe contrast is recognized as a decrease in the image quality when thecontrast decreases to about 2/3. The procedure of this experiment willbe described below.

A motion picture was projected for 10 minutes with the contrast of thedisplay screen held constant, and then for 5 minutes after the contrastwas changed. Thereafter, information on whether the contrast change wasrecognized as degradation in the image quality was obtained through aquestionnaire, thereby calculating a threshold value at which the changewas recognized as degradation.

As shown in FIG. 34, the experiment was conducted by setting the initialvalue of the threshold value of the contrast to 200:1, 100:1, 50:1, and20:1. As a result, when the initial value was 100:1 or smaller, thechange began to be recognized as degradation in the image quality fromabout 85% of the initial value and was recognized as degradation in theimage quality for most samples at about 65% of the initial value.Therefore, the aperture control must be done at an accuracy with which areduction in the contrast is 40% or lower for displayed images of thesame luminance.

That is, it is necessary to control the solid angle within the range ofabout ±30%. For example, when circular aperture stops are used, acontrol accuracy of ±0.15 θ is required with respect to an apex angle θof a conical light flux. For example, if a change in the angle ofcollection of the aperture stop is θ=3° to 10°, the accuracy of theaperture stop angle of collection must be controlled within the range of±0.5° to ±1.5°.

An aperture adjustment margin with respect to the screen luminance(unit:cd/m²) will be described below.

If the screen luminance (unit:cd/m²) resulting from external light ishigh, a change in the contrast with respect to the angle of collectionis modest compared to that when there is no external light.Consequently, the necessary control accuracy decreases as the externallight increases.

As illustrated in FIG. 5, the optimized contrast is nearly one half ofan original contrast free from the influence of external light. Forexample, when the illuminance of a room is 0 lux, the optimum contrastis 50 in correspondence with the aperture stop state in which thecontrast is 100. Similarly, a contrast corresponding to a contrast of 30is about 15.

It is therefore considered that when the angle of collection of theaperture stop is optimized with respect to external light, the luminanceof the screen in an original black display is almost equal to theluminance of the screen under the influence of external light.

If the contrast is shifted from its optimum value due to an erroroccurring in the control of the angle of collection, the contrast isgiven by the following equation. ##EQU6##

Note that S_(ext) indicates a value obtained by converting the luminanceof the external light into the angle of collection of the aperture stopin a black display of the projection display apparatus. Since theoriginal black display of the projection display apparatus is equal tothe influence of the luminance of the external light, S_(ext) =S₀.

As can be seen from the above equation, the margin is doubled. However,when the presence of a margin pertaining to the agreement between thetwo aperture stops discussed above is taken into consideration, an errorof the aperture stop angle of collection to be controlled with respectto the external conditions is one half the margin. Therefore, in thiscase a margin concerning the agreement between the angle of collectionsof the two aperture stops is also required as the margin.

There will be described the fifth aspect of the present invention withreference to FIG. 41. The fundamental structure of the apparatus shownin FIG. 6A is substantially the same as that shown in FIG. 28.

As shown in FIG. 41, the display apparatus has a driving voltage supplycircuit 731 for supplying the video signals (V_(SR)), (V_(SG)), and(V_(SB)) to the liquid crystal panels 301-R, 301-G, and 301-B,respectively, and a polarity inverting/noninverting amplifier 751.

In the driving voltage supply circuit 731, as illustrated in FIG. 33, ananalog-to-digital converter 733 converts the input brightness signal(BS) into a digital signal. A gamma-correcting circuit 734 constitutedby a ROM performs gamma correction for the brightness signal (BS) in thedigital signal form and supplies the gamma-corrected digital brightnesssignal to one input terminal of an adder 735. In addition, on the basisof the environmental luminance signal (ES) from the photosensor, alight-leak-compensating circuit 744 constituted by a ROM supplies, tothe other input terminal of the adder 745, compensation data forcorrecting the potential lowering due to light leak current (I_(off)) ofTFT 541 constituting the liquid crystal panel 301-G. The adder 745outputs the sum of the digital brightness signal and the compensationdata to a digital-to-analog converter 747. The digital-to-analogconverter 747 outputs an analog signal to the polarityinverting/noninverting amplifier 741.

The polarity inverting/noninverting amplifier 741 amplifies the analogsignal to have a signal level required for the liquid crystal panel301-G and converts the signal into a video signal (V_(SG)) which invertsits polarity with respect to the reference voltage at a predeterminedfrequency for, e.g., each field period or each scan period. Theamplifier 741 supplies the video signal (V_(SG)) to the liquid crystalpanel 301-G.

Similarly, video signals (V_(SR)) and (V_(SB)) corrected based on theenvironmental luminance signal (ES) supplied from the photosensor 711are supplied to the other liquid crystal panels 201-R and 201-B. In thiscase, the aperture stop control means 721 controls the aperture D1 ofthe first aperture stop means 151 and the aperture D2 of the secondaperture stop means 503, such that the light-collecting angles Ω₁ and Ω₂increase as the luminance of the screen increases. At the same time, thedrive voltage supplying circuit 731 also control the apertures D₁ and D₂so that the video signals V_(SR), V_(SG) and V_(SB) supplied to theliquid crystal panels 201-R, 201-G and 201-B may increase in magnitude.

The operation of the projection type display apparatus described abovewill now be explained briefly.

First, the photosensor 711 detects the luminance (in cd/m²) on thescreen, generating an environmental luminance signal (ES). From thesignal (ES) the aperture stop control means 721 determines apertures D₁and D₂ appropriate for the first aperture stop means 151 and the secondaperture means 503. The higher the screen illuminance, the greater theapertures D₁ and D₂.

In the case where the drive voltage supplying circuit 731 does notoptimize the video signals (V_(SR)), (V_(SG)) and (V_(SB)) because thescreen illuminance is as low as 30 lux, the converting angles Ω₁ and Ω₂are set at 1.1×10⁻³ sr, achieving contrast ratio of 70:1. If screenilluminance is as high as 200 lux, the angles Ω₁ and Ω₂ are set at8.6×10⁻³ sr, providing high display peak luminance flux of 75 lm, muchhigher than the display peak luminance flux of 18 lm attained when theangles Ω₁ and Ω₂ are limited to 1.1×10⁻³ sr.

In this embodiment, when the screen illuminance is 200 lux, the drivevoltage supplying circuit 731 increases in accordance with the signal(ES) the magnitudes of the video signals (V_(SR)), (V_(SG)) and (V_(SB))by about 10%, as compared with the case where the screen luminance is 30lux.

When the video signals (V_(SR)), (V_(SG)) and (V_(SB)) were optimized inthis way, both the contrast ratio and the display luminance increasedabout 10% from the values specified above, when the screen illuminanceis 200 lux.

In this embodiment, the contrast ratio and the display luminance areprevented from degrading despite the light-leakage current Ioff whichflows through the FET 241 due to an increase in the angle of collectionΩ₁. The embodiment can therefore attains sufficient display quality.

In the present embodiment, the second aperture stop means 503 is drivenin complete interlock with the first aperture stop means 151 so that itsaperture D₂ may equal to the aperture D₁ of the first aperture stopmeans 151. Nonetheless, the the second aperture stop means 503 may bedriven to have its aperture changed differently from that of the firstaperture stop means 151.

In this embodiment, three aperture stop means may be provided for theliquid crystal panels 201-R, 201-G and 201-B, respectively, so that theangle of collection Ω₁ can be controlled for each color independently.In the embodiment, the aperture stop control means 721 controls theangle of collection Ω₁ for the first and second aperture stop means 151and 503 in accordance with the environmental luminance signal (ES).Nevertheless, the first aperture stop means 151 and the second aperturestop means 503 may be controlled by a brightness signal (BS) input tothe drive voltage supplying circuit 741. Alternatively, theenvironmental luminance signal (ES) and the brightness signal (BS) maybe used in combination to control the first and second aperture stopmeans 151 and 503. For example, the angles Ω₁ and Ω₂ may changed inaccordance with the difference between the average magnitude of thebrightness signal (BS) and the blank level (black level) of thebrightness signal (BS). More specifically, the angles Ω₁ and Ω₂ are setat small values when the difference is small, and at large values whenthe difference is large. High-quality images can therefore be displayed,irrespective of their display luminance.

In this embodiment, the photosensor 711 monitors the luminance of thescreen. Instead, it may monitor the environmental illuminance (lux).

This embodiment is a three-plate projection type display apparatus 100.The present invention is not limited to this type of a projectiondisplay apparatus. Rather, it can be applied to a single-plateprojection display apparatus which has a liquid crystal panel with colorfilters of three primary colors which are arranged in a striped ormosaic pattern.

Since the micro-lens array substrate 411 is used in combination with theliquid crystal panels 201-R, 201-G and 201-B, the effective aperture ofeach panel is great enough to enhance the effective contrast and thedisplay luminance on the screen.

The liquid crystal panel 201-G will now be described with reference toFIG. 43. A microlens array substrate 411 is adhered onto the majorsurface of the substrate 311 constituting the panel 201-G, with anadhesive layer 410 interposed between them. The substrate 411 includes agroup of focusing lenses 413 corresponding to the respective displaypicture elements, and the focal point of each lens 413 is set so as tofall within the glass substrate 210.

If the liquid crystal panels 201-R, 201-G and 201-B are constituted asdescribed above, the light, which was shielded by the light-shieldinglayer 313, can be utilized effectively; therefore, the effectiveaperture rate of each of the panels 201-R, 201-G and 201-B can beincreased and, even if the angle of collections Ω₁ and Ω₂ are narrowed,a considerable peak flux is obtained, and the displayed luminance isprevented from decreasing.

When the microlens array substrate 411 is provided on the light-incomingside of the liquid crystal panel 201-G as in the above embodiment, it ispreferable that the aperture diameter D₂ of the second aperture stopmeans 503 be set larger than the diameter D₁ of the first aperture stopmeans 151. This is because the light focused by the microlens isscattered afterward.

More specifically, when the microlens array substrate 411 is arranged ononly the light-incoming side as in the above embodiment, the efficiencyof use of the light focused in the vicinity of the light-shielding layer313 by the microlens array substrate 411, is likely to decrease, sincethe light is scattered afterward. It is thus important to determine thefocal point of each focusing lens 413 of the substrate 411. In otherwords, the focal point has to be set such that the scattered light fallswithin the projection lens 505. To miniaturize the projection lens 505,the focal length of each focusing lens 413 should be greater and thenumerical aperture thereof should be smaller. However, as the distancebetween the focal point and the light-incoming side increases, theeffect of the microlens array substrate 411 lowers and so does thedegree of increase in the effective aperture rate of each liquid crystalpanel. Consequently, it is advisable to set the focal point of eachfocusing lens 413 within the substrate 210 on the light-outcoming sideor slightly outside the substrate 210 rather than within the PD-typeliquid crystal layer 410.

According to the above embodiment, the servo motors are used as thefirst and second aperture stop 151 and 503, as shown in FIG. 41, and thediameters D₁ and D₂ of circular apertures are varied by the servomotors. However, the shape of the apertures can be rectangular,elliptic, and the like. Furthermore, the light-shielding plate forshielding the upper and lower portions of the apertures by the servomotors can be movably constituted.

In the above embodiment, the angle of collections Ω₁ and Ω₂ arecontrolled by controlling the aperture diameters D₁ and D₂ of the firstand second aperture stop means 151 and 503 by the first aperture stopcontrol means 721. However, the angle of collections can be controlledby controlling the aperture diameters by moving the first and secondaperture stop means along the optical axis of the light source 111.

The active matrix type liquid crystal panel in which a switching elementof TFT is provided for each of the display picture elements, has beendescribed as a liquid crystal panel of the above embodiment. The TFT canbe formed mainly of a polycrystalline silicon film or a monocrystallinesilicon film.

The sixth aspect of the present invention will be described below.

This aspect relates to a simplified projection display apparatus. Tosimplify the operation, after a power supply is turned on the setting ofaperture stops is automatically optimized in accordance with the ambientluminance. FIG. 35 shows one embodiment of this simplified projectiondisplay apparatus. In this display apparatus, by assuming the size andthe gain of a screen in advance, the luminance of the screen is directlymeasured by measuring a projected bundle of rays with no projected lightpresent. This makes it possible to rapidly determine the optimum angleof the aperture stops. This determination of the aperture stop angle ofcollection is done automatically at the same time the power supply ofthe projection display apparatus is turned on.

This display apparatus includes an optical path interrupting unit 901 inorder to measure the luminance of the screen with no projected lightfrom the projection display apparatus. As in FIG. 35, this optical pathinterrupting unit 901 is arranged on the exit side of the panel. Theunit 901 is controlled by an optical path interrupting unit controller900 to perform switching between the interrupted and the noninterruptedstates of the light from a light source at the entrance of projectionlenses.

FIG. 36 is a flow chart showing the processing of automaticallydetermining the angle of collection after an ON operation of the powersupply in the display apparatus illustrated in FIG. 35. It takes a fewminutes to completely turn on a metal halide lamp as the light source.In the display apparatus shown in FIG. 35, the luminance of the screenresulting from the brightness of a room is measured with the opticalpath interrupted by the optical path interrupting unit. Therefore, auser need not wait until the luminance of the lamp reaches a steadystate. In this processing, the lamp is turned on (application of a highvoltage and discharge are started) first, but the luminance of thescreen is measured to determine the angle of collection of the aperturestops without waiting until the lamp is completely turned on.

FIG. 37 shows another embodiment of the simplified projection displayapparatus. To simplify the operation, this display apparatus includes anencoder 910 in projection lenses to detect the focal position of theprojection lenses. A projection distance detector 920 allows a CPU todetect the output from the encoder 910. Therefore, the screen size canbe detected by the projection distance detector 920.

In the display apparatus illustrated in FIG. 37, a projectioncoefficient q in the following equation is automatically calculated byassuming the screen gain is 1.5. Consequently, since only L₀ is unknown,luminance L₀ of a room can be obtained by measuring screen luminance L,in the state of I=0, by an optical path interrupting unit.

    L=ql+L.sub.0

FIG. 38 is a flow chart showing the processing of automaticallydetermining the angle of collection in the display apparatus of FIG. 37.In this display apparatus the screen size is detected by the focalposition of the projection lenses. Therefore, after the focal point of aprojected image is obtained, optimization of the aperture stops isexecuted by depressing an aperture stop optimization switch.

The display quality of this projection type display apparatus changesdepending on the luminance of black level of a screen which depends onvarious environmental luminance levels as of a projection location and atime zone. In correction using the display quality by the environmentalluminance in a front projection type display apparatus, a large distanceis present between the front projection type display apparatus and thescreen, and the environmental luminance of a location at which theprojection display apparatus is located is different from the luminancearound the screen. In practice, the illuminance and luminance of theillumination which are determined by the screen surface almostvertically standing and the screen gain contribute to the displayquality. For this reason, the display quality is greatly influenced byenvironmental illumination conditions around the screen and externallight from a room window. It is therefore important to control thedisplay quality of the projection type display apparatus by directlymonitoring the screen surface luminance changed by the environmentalluminance. In an application field such as a conference, theenvironmental luminance of an entire room or part of the room ispreferably controlled in accordance with a use condition, or the displayquality is preferably adjusted in accordance with the screen surfaceluminance at the final stage because the environmental luminance aroundthe screen changes from an adjustment stage to the final operationstage. For this purpose, it is important that a photosensor formonitoring the luminance of black display level and the contrast levelon the screen surface of the display apparatus is arranged in theprojection display apparatus, thereby performing a higher-quality imagedisplay.

The embodiments of front projection type display apparatuses using areflection screen have been described above. However, the presentinvention is not limited to these embodiments but can be applied to rearprojection type display apparatuses as shown in FIGS. 39 and 40.

In a rear projection type display apparatus illustrated in FIGS. 39 and40, the light from an optical system 950 is reflected by a first mirror960 and a second mirror 970 and projected on a transmission screen 980.The apparatus also includes an optical sensor for measuring the screenluminance resulting from external light. In the front projection typedisplay apparatuses discussed above, the optical sensor is arranged onthe side of the projection lenses and directed to the screen to measurethe intensity of the reflected light from the screen. In this rearprojection type display apparatus, as illustrated in FIGS. 39 and 40,the optical sensor can be mounted on the peripheral portion of thescreen, like a sensor A or B, to measure the illuminance. In rearprojection type display apparatuses, the characteristics of the screenare already determined in many cases. Therefore, it is possible toestimate the screen luminance from the illuminance.

The optical sensor can also be arranged inside the screen, like a sensorC, to measure the luminance of external light passing through thescreen. In order for these sensors to be able to measure only theinfluence of pure external light, these sensors, of course, must be soarranged that the light from the projection optical system is notincident on the sensors.

In addition, in rear projection type display apparatuses the projectioncoefficient q in the above equation is already determined. Therefore, anoptimum aperture stop angle of collection can be determined only bymeasuring luminance L₀ of external light.

Additional advantages and modifications will readily occur to thoseskilled in the art. Therefore, the invention in its broader aspects isnot limited to the specific details, and representative devices shownand described herein. Accordingly, various modifications may be madewithout departing from the spirit or scope of the general inventiveconcept as defined by the appended claims and their equivalents.

What is claimed is:
 1. A display apparatus comprising:a light source; amodulating device for modulating light emitted from said light source;first aperture stop means, arranged between said light source and saidmodulating device, for limiting a bundle of rays to be incident on saidmodulating device from said light source, said first aperture stop meanshaving an aperture with a variable size; a display screen onto whichexit light from said modulating device is projected; a projectionoptical system for projecting the exit light from said modulating deviceonto said display screen; second aperture stop means, arranged betweensaid modulating device and said projection optical system, for limitinga bundle of rays to be incident on said projection optical system fromsaid modulating device, said second aperture stop means having anaperture with a variable size; aperture control means for controllingthe size of the aperture of at least one of said first and secondaperture stop means; and liquid crystal driving means for supplying tosaid modulating device a picture signal controlled on the basis of thesize of the aperture, which is controlled by said aperture controlmeans, of at least one of said first and second aperture stop means. 2.An apparatus according to claim 1, further comprising a photosensor fordetecting a display luminance on said display screen,wherein saidaperture control means controls the size of the aperture of at least oneof said first and second aperture stop means on the basis of a displayluminance signal from said photosensor.
 3. An apparatus according toclaim 2, whereinsaid aperture control means controls the size of theaperture of at least one of said first and second aperture stop means onthe basis of the display luminance signal and a contrast signal fromsaid photosensor.
 4. An apparatus according to claim 3, whereinsaidphotosensor detects a white level display luminance on said displayscreen on which a white image is displayed by a white image signalsupplied from said liquid crystal driving means to said modulatingdevice, and detects a black level display luminance on said displayscreen on which a black image is displayed by a black image signalsupplied from said liquid crystal display means to said modulatingdevice, and said aperture control means controls the size of theaperture of at least one of said first and second aperture stop means onthe basis of a contrast calculated from a ratio of the white leveldisplay luminance to the black level display luminance detected by saidphotosensor.
 5. An apparatus according to claim 4, wherein said aperturecontrol means controls the size of the aperture of at least one of saidfirst and second aperture stop means such that the contrast ismaximized.
 6. An apparatus according to claim 2, whereinsaid photosensordetects an environmental luminance on said display screen resulting fromlight of an environment in which said display apparatus is placed, andsaid aperture control means controls the size of the aperture of atleast one of said first and second aperture stop means on the basis ofan environmental luminance signal and the display luminance signal fromsaid photosensor.
 7. An apparatus according to claim 1, wherein saidaperture control means controls angle of collections of said first andsecond aperture stop means.
 8. An apparatus according to claim 7,wherein said aperture control means controls the size of the aperture ofat least one of said first and second aperture stop means such that theangle of collections of said first and second aperture stop means aresubstantially equal to each other.
 9. An apparatus according to claim 1,further comprising:a spheroidal mirror arranged near said light sourceand having a focal point at the position of said light source; and acollimator optical system for guiding light reflected by said spheroidalmirror to said modulating device.
 10. An apparatus according to claim 1,wherein said modulating device is one selected from the group consistingof a dispersion type liquid crystal device, and a slant field effectliquid crystal diffraction grating.
 11. An apparatus according to claim10, wherein said dispersion type modulating device is one of a polymerdispersion type modulating device and a fine-particle dispersion typemodulating device.